The Effect of Soil Organic Carbon Amendments on Nitrogen Dynamics after Broccoli (Brassica
oleracea L. var. Italica) Production
by
Katelyn Amber Congreves
A Thesis
presented to
The University of Guelph
In partial fulfillment of requirements
for the degree of
Doctor of Philosophy
in
Land Resource Science
Guelph, Ontario, Canada
© Katelyn A. Congreves, April, 2014
ABSTRACT
THE EFFECT OF SOIL ORGANIC CARBON AMENDMENTS ON NITROGEN
DYNAMICS AFTER BROCCOLI (BRASSICA OLERACEA L. VAR. ITALICA)
PRODUCTION
Katelyn A. Congreves
University of Guelph, 2014
Advisor:
Dr. Laura L. Van Eerd
After broccoli (Brassica oleracea L.) harvest over 400 kg N ha-1 typically remains in the field as
readily mineralizable crop residues and soil inorganic N which is susceptible to post-harvest
losses. Therefore, the goal of this research was to develop a best management practice (BMP) to
minimize potential N losses after broccoli production. Organic C amendments (OCAs) may
reduce N losses via immobilization, but for the development of an effective BMP research must
evaluate the synchrony of OCA and crop residue decomposition rates, quantity of net N
immobilization and fate of crop residue-N, and potential effects of OCA on subsequent crop
production. Therefore, N dynamics were evaluated via incubation, field, and tracer studies after
broccoli (var. italica) harvest with soil OCAs. In the 56 d laboratory trial, broccoli crop residue
or fertilizer-N was incorporated with either wheat straw, yard waste, or used cooking oil. In the
field during 2009-2010 and 2010-2011, wheat straw, yard waste, or used cooking oil were
applied in a broccoli-spring wheat (Triticum durum L.) rotation. From broccoli to spring wheat in
2011-2012, 15N enrichments were used to trace the fate of residual-N into soil and plant pools,
with or without used cooking oil. Net N immobilization was observed with all OCAs in the
incubation study, but used cooking oil had a synchronous decomposition rate with broccoli
residue and a synergistic effect on N immobilization and microbial activity. In the field studies
and compared to the typical practice, wheat straw and yard waste reduced spring wheat yield or
had an inconsistent effect in decreasing potential N losses, thus are not recommended as a BMP.
In the field and tracer studies relative to no amendment, used cooking oil consistently reduced
potential N losses by up to 112 kg N ha-1 and increased recovery of broccoli crop residue-N in
soil total, mineral, and organic N fractions by 190-209%, without affecting subsequent spring
wheat yield or N content. It is therefore recommended that growers apply used cooking oil as a
BMP at broccoli harvest to minimize potential above-ground crop residue-N losses and to
increase the soil organic N fraction.
PREFACE
The presented dissertation is organized as a series of manuscripts published or submitted for
publication in peer-reviewed scientific journals. As a result, there is some unavoidable repetition
among research sections. Each manuscript was written by Katelyn A. Congreves. The references
for the publications are listed below:
Congreves, K.A., Voroney, R.P., O’Halloran, I.P., Van Eerd, L.L. 2013. Broccoli residuederived nitrogen immobilization following amendments of organic carbon: An incubation study.
Canadian Journal of Soil Science. 93: 23-31.
Congreves, K.A., Vyn, R.J., Van Eerd, L.L. 2013. Evaluation of post-harvest organic carbon
amendments as a strategy to minimize nitrogen losses in cole crop production. Agronomy. 3:
181-199.
Congreves, K.A., Voroney, R.P., Van Eerd, L.L. Amending soil with used cooking oil to reduce
nitrogen losses after cole crop harvest: A 15N study. Submitted to the Soil Science Society of
America Journal Jan 24, 2014.
iv
ACKNOWLEDGEMENTS
I would like to express deep gratitude to my advisor Dr. Laura Van Eerd for her expertise and
support throughout my degree. Thank you, Laura, for providing me with the best environment
and tools to develop research and writing skills. I sincerely thank Dr. Paul Voroney for sharing
his expertise, enthusiasm for soil science, and for providing positive encouragement. Thank you
to my advisory committee members Dr. Ivan O’Halloran and Dr. Bill Deen for your excellent
questions and constructive criticism on my research.
To the exam committee members including Dr. Laura Van Eerd, Dr. Paul Voroney, Dr. Ralph
Martin, Dr. Diane Knight, and Dr. Claudia Wagner-Riddle, I thank you for your time,
knowledge, and discussion during the defense. I feel honoured to work with such experts in
agricultural science.
I greatly acknowledge the tireless and kind help in the field and laboratory from Mike Zink and
the summer students on the soil crew who provided valuable hours of labour throughout my
projects at Ridgetown Campus. Special thanks to Myles Stocki for isotope analysis and his
helpful/enjoyable email correspondence.
I thank the following sponsors for their support: the Natural Sciences and Engineering Research
Council of Canada, the Ontario Ministry of Agriculture and Food, the Ontario Ministry of Rural
Affairs, the Department of Agriculture and Agri-Food Canada, the Soil Science Society of
America, and the Canadian Society of Soil Science.
For always caring, encouraging, and supporting, I thank Michael, Carolyn, Shawn, Matthew, and
Mikaela Congreves. Lastly, I would like to express my profound gratitude to my husband,
Jonathan Gorham, whose unconditional love, support, and positive outlook has helped me each
step along my journey. Jon, I dedicate my dissertation to you.
v
TABLE OF CONTENTS
PREFACE ..................................................................................................................................... iv
ACKNOWLEDGEMENTS ......................................................................................................... v
TABLE OF CONTENTS ............................................................................................................ vi
LIST OF TABLES ....................................................................................................................... ix
LIST OF FIGURES ................................................................................................................... xiv
ABBREVIATIONS .................................................................................................................... xvi
1
INTRODUCTION ............................................................................................................... 16
1.1
NITROGEN CYCLE ..................................................................................................... 16
1.1.1
Nitrogen Fixation .................................................................................................... 3
1.1.2
Nitrogen Mineralization .......................................................................................... 5
1.1.3
Nitrogen Immobilization......................................................................................... 7
1.1.4
Nitrification ............................................................................................................. 9
1.1.5
Nitrate Leaching.................................................................................................... 10
1.1.6
Surface Runoff ...................................................................................................... 12
1.1.7
Denitrification ....................................................................................................... 13
1.1.8
Ammonia Volatilization and Ammonium Adsorption ......................................... 14
1.1.9
Plant Uptake .......................................................................................................... 14
1.2
NITROGEN LOSSES .................................................................................................... 16
1.2.1
Consequences of Nitrogen Losses ........................................................................ 16
1.2.2
Seasonal Risks of Soil Nitrogen Losses................................................................ 17
1.3
COLE CROP PRODUCTION AND NITROGEN MANAGEMENT .......................... 19
1.3.1
General Cole Crop Production in Ontario ............................................................ 19
1.3.2
Nitrogen Use Efficiency ....................................................................................... 20
1.3.3
Cover Crops .......................................................................................................... 22
1.3.4
Crop Residue Management ................................................................................... 22
1.4
ORGANIC CARBON AMENDMENTS ....................................................................... 24
1.5
CONCLUSIONS ............................................................................................................ 25
1.6
RESEARCH OBJECTIVES .......................................................................................... 26
2 BROCCOLI RESIDUE-DERIVED NITROGEN IMMOBILIZATION FOLLOWING
AMENDMENTS OF ORGANIC CARBON: AN INCUBATION STUDY. ......................... 28
vi
2.1
ABSTRACT ................................................................................................................... 28
2.2
INTRODUCTION .......................................................................................................... 29
2.3
MATERIALS AND METHODS ................................................................................... 31
2.3.1
Nitrogen Mineralization ........................................................................................ 33
2.3.2
Carbon Mineralization .......................................................................................... 35
2.3.3
Statistical Analyses ............................................................................................... 35
2.4
RESULTS AND DISCUSSION .................................................................................... 37
2.4.1
Mineral Nitrogen Concentrations ......................................................................... 37
2.4.2
Net Nitrogen Mineralization ................................................................................. 38
2.4.1
Net Carbon Mineralization ................................................................................... 44
2.4.2
Overall Nitrogen and Carbon Dynamics............................................................... 49
2.5
CONCLUSION .............................................................................................................. 49
3 EVALUATION OF POST-HARVEST ORGANIC CARBON AMENDMENTS AS A
STRATEGY TO MINIMIZE NITROGEN LOSSES IN COLE CROP PRODUCTION. .. 51
3.1
ABSTRACT ................................................................................................................... 51
3.2
INTRODUCTION .......................................................................................................... 52
3.3
MATERIALS AND METHODS ................................................................................... 55
3.3.1
Nitrogen Measurements ........................................................................................ 60
3.3.2
Economic Analysis ............................................................................................... 60
3.3.3
Statistical Analysis ................................................................................................ 61
3.4
RESULTS AND DISCUSSION .................................................................................... 62
3.4.1
Broccoli Harvest ................................................................................................... 62
3.4.2
Soil Mineral Nitrogen in Autumn ......................................................................... 63
3.4.3
Soil Mineral Nitrogen in the Subsequent Spring and Summer ............................. 69
3.4.4
Spring Wheat Production ...................................................................................... 70
3.4.5
Economic Analysis ............................................................................................... 77
3.5
CONCLUSION .............................................................................................................. 79
4 AMENDING SOIL WITH USED COOKING OIL TO REDUCE NITROGEN
LOSSES AFTER COLE CROP HARVEST: A 15N STUDY. ............................................... 81
4.1
ABSTRACT ................................................................................................................... 81
4.1
INTRODUCTION .......................................................................................................... 82
vii
4.2
MATERIALS AND METHODS ................................................................................... 84
4.2.1
Experimental Design ............................................................................................. 85
4.2.2
Nitrogen Measurements ........................................................................................ 88
4.2.3
Calculations........................................................................................................... 89
4.2.4
Statistical Analysis ................................................................................................ 91
4.3
RESULTS AND DISCUSSION .................................................................................... 93
4.3.1
Fate of Fertilizer Derived Nitrogen at Broccoli Harvest ...................................... 93
4.3.2
Fate of Above-ground Broccoli Crop Residue Derived Nitrogen ........................ 96
4.3.3
Fate of Residual Fertilizer or Broccoli Root Derived Nitrogen .......................... 108
4.4
CONCLUSIONS .......................................................................................................... 112
5
CONCLUSIONS AND RECOMMENDATIONS .......................................................... 114
6
REFERENCES .................................................................................................................. 118
7
APPENDICES .................................................................................................................... 136
APPENDIX A ALTERNATIVE STRATEGIES TO REDUCE NITROGEN LOSSES
AFTER COLE CROP HARVEST.............................................................................. 136
APPENDIX B STATISTICAL TABLES FOR DATA IN CHAPTER 2 .................. 141
APPENDIX C STATISTICAL TABLES FOR DATA IN CHAPTER 3 .................. 152
APPENDIX D STATISTICAL TABLES FOR DATA IN CHAPTER 4 .................. 158
viii
LIST OF TABLES
Table 1.1 Types of microbial enzymes involved in decomposition. * ........................................... 6
Table 2.1 The C:N ratio and N content (mg kg-1 dry matter) of the amendments z...................... 32
Table 2.2 The effect of organic carbon amendment with broccoli residue-derived N or fertilizerderived N on net N and C mineralization at 4 or 56 d of incubation z. ......................................... 39
Table 2.3 Effect of organic carbon amendment with broccoli residue-derived N or fertilizerderived N on net N mineralization in the 56 d incubation. ........................................................... 43
Table 3.1 Initial soil characteristics of the experimental sites prior to broccoli transplanting in
2009 and 2010. .............................................................................................................................. 56
Table 3.2 Monthly total precipitation and mean temperature and the 30-yr mean at Ridgetown,
ON during 2009–2011. ................................................................................................................. 58
Table 3.3 The effect of post-broccoli-harvest treatments on 2010 and 2011 spring wheat profit
margins ($ ha−1) subsequent to the 2009 and 2010 early- and late-harvested broccoli. ............... 78
Table 4.1 Monthly temperature (°C) and precipitation (mm) at Ridgetown, ON during the
broccoli - spring wheat crop rotation. ........................................................................................... 85
Table 4.2 The fate of fertilizer-derived N at harvest of early (Aug) and late (Sept) broccoli
production in 2011†. ..................................................................................................................... 94
Table 4.3 The fate of above-ground crop residue-derived N with vs. without used cooking oil
two weeks after early and late broccoli production (Sept or Oct 2011) and prior to soil freeze-up
(Nov 2011) . .................................................................................................................................. 98
Table 4.4 The 0-60 cm nitrate-nitrogen (NO3--N) and soil mineral N (SMN) from broccoli
production (early and late pooled) in field plots during autumn (Sept/Oct 2011), at spring wheat
planting (Apr 2012) †, and at spring wheat harvest (July 2012)††. ............................................. 99
Table 4.5 The fate of above-ground crop residue-derived N with vs. without used cooking oil
after early and late broccoli production in soil at spring wheat planting (Apr 2012) and harvest
(July 2012). ................................................................................................................................. 102
ix
Table 4.6 The fate of above-ground crop residue-derived N with vs. without used cooking oil
after early and late broccoli production in spring wheat plant tissue at harvest (Jul 2012)†. ..... 105
Table 4.7 2012 spring wheat harvest parameters following after early and late broccoli
production and field plot treatments†. ........................................................................................ 106
Table 4.8 The fate of residual fertilizer or broccoli root-derived N with vs. without used cooking
oil two weeks after early and late broccoli production (Sept or Oct 2011), prior to soil freeze-up
(Nov 2011), at spring wheat planting (Apr 2012) and spring wheat harvest (Jul 2012). ........... 110
Table 4.9 The fate of residual fertilizer or broccoli root-derived N with vs. without used cooking
oil after early and late broccoli production in spring wheat plant tissue at harvest (Jul 2012)†. 112
Table A.2 Treatment list of strategies to reduce nitrogen loss. .................................................. 136
Table B.1 Analysis of variance using SAS Proc Mixed for the fixed effects of cumulative net N
and C mineralization shown in sections 2.4.1. and 2.4.2. ........................................................... 141
Table B.2 Least significance difference test using SAS Proc Mixed of treatment x day effect
sliced by sample day for cumulative net N mineralization (mg N kg-1), shown in Figure 2.1.
Statistical differences among rows indicated by different letters and case (P<0.05). ................ 143
Table B.3 Least significance difference test using SAS Proc Mixed of treatment x day effect
sliced by sample day for cumulative net C mineralization (mg N kg-1), shown in Figure 2.2.
Statistical differences among rows indicated by different letters and case (P<0.05). ................ 146
Table C.1 SAS Proc Mixed analysis of variance probability values of 0-30 cm SMN 2009 and
2010 after early and late broccoli harvest systems in autumn, discussed in section 3.4.2. ........ 152
Table C.2 SAS Proc Mixed analysis of variance probability values of 0-30 cm SMN 2009 after
early broccoli harvest in autumn, shown in Figure 3.1. .............................................................. 152
Table C.3 SAS Proc Mixed analysis of variance probability values of 0-30 cm SMN 2010 after
early broccoli harvest in autumn, shown in Figure 3.1. .............................................................. 153
Table C.4 SAS Proc Mixed analysis of variance probability values 0-30 cm SMN 2009 after late
broccoli harvest in autumn, shown in Figure 3.1. ....................................................................... 153
Table C.5 SAS Proc Mixed analysis of variance probability values 0-30 cm SMN 2010 after late
broccoli harvest in autumn, shown in Figure 3.1. ....................................................................... 153
x
Table C.6 SAS Proc Mixed analysis of variance probability values of SMN at spring wheat
planting 2010 and 2011 after early and late broccoli harvest systems in 2009 and 2010, shown in
Figure 3.1. ................................................................................................................................... 154
Table C.7 SAS Proc Mixed analysis of variance probability values of plant available N (0-90 cm
SMN, straw N, and grain N) at spring wheat harvest in 2010 and 2011 after early and late
broccoli harvest systems in 2009 and 2010, shown in Figure 3.1 and Figure 3.2. ..................... 155
Table C.8 SAS Proc Mixed analysis of variance probability values of grain yield at spring wheat
harvest in 2010 and 2011 after early and late broccoli harvest systems in 2009 and 2010, shown
in Figure 3.3. ............................................................................................................................... 156
Table C.9 SAS Proc Mixed analysis of variance probability values of plant biomass (straw and
grain) at spring wheat harvest in 2010and 2011 after early and late broccoli harvest systems in
2009 and 2010, shown in Figure 3.3. .......................................................................................... 157
Table D.1 SAS Proc Mixed analysis of variance probability values of fertilizer-derived N in
broccoli plant (head, stem, leaf) and soil pools (0-30 and 30-60 cm total, mineral, organic) at
early and late broccoli harvest systems 2011, shown in Table 4.2. ............................................ 158
Table D.2 SAS Proc Mixed analysis of variance probability values of 0-60 cm soil nitrate-N
(NO3--N) and soil mineral N (SMN) after early and late broccoli harvest systems from 2011 to
2012 sampling, shown in Table 4.4. ........................................................................................... 159
Table D.3 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil Nderived from above-ground 15N enriched broccoli crop residue after early and late broccoli
harvest systems in autumn 2011, shown in Table 4.3................................................................. 160
Table D.4 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil N
recovered from above-ground 15N enriched broccoli crop residue after early and late broccoli
harvest systems in autumn 2011, shown in Figure 4.3. .............................................................. 161
Table D.5 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil
total N (14N and 15N) in autumn 2011, after above-ground 15N enriched broccoli crop residue was
incorporated at early and late broccoli harvest systems, shown in Figure 4.3............................ 162
Table D.6 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil Nderived from 15N enriched broccoli crop residue after early and late broccoli harvest systems at
spring wheat planting (Apr) and harvest (Jul) in 2012, shown in Table 4.5............................... 163
xi
Table D.7 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil N
recovered from 15N enriched broccoli crop residue after early and late broccoli harvest systems at
spring wheat planting (Apr) and harvest (July) in 2012, shown in Table 4.5............................. 164
Table D.8 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil
total N (14N and 15N) at spring wheat planting (Apr) and harvest (July) 2012, after above-ground
15
N enriched broccoli crop residue was incorporated at early and late broccoli harvest systems,
shown in Table 4.5. ..................................................................................................................... 165
Table D.9 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat yield or
biomass after above-ground 15N enriched broccoli crop residue was incorporated at early and late
broccoli harvest systems, shown in Table 4.6............................................................................. 166
Table D.10 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat Nderived from broccoli crop residue after above-ground 15N enriched broccoli crop residue was
incorporated at early and late broccoli harvest systems, shown in Table 4.6. ............................ 166
Table D.11 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat N
recovered from broccoli crop residue after above-ground 15N enriched broccoli crop residue was
incorporated at early and late broccoli harvest systems, shown in Table 4.6. ............................ 167
Table D.12 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat total N
(14N and 15N) after above-ground 15N enriched broccoli crop residue was incorporated at early
and late broccoli harvest systems, shown in Table 4.6. .............................................................. 167
Table D.13 SAS Proc Mixed analysis of variance probability values of spring wheat harvest
parameters in 2012, following early and late broccoli systems in the non-tracer trial, shown in
Table 4.7. .................................................................................................................................... 168
Table D.14 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil
N-derived from 15N enriched residual fertilizer or broccoli roots after early and late broccoli
harvest systems in autumn (Sept to Nov 2011) , spring wheat planting (Apr 2011) and harvest
(July 2012), shown in Table 4.8.................................................................................................. 169
Table D.15 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil
total N (14N and 15N) after early and late broccoli systems with 15N enriched residual fertilizer or
broccoli roots in autumn (Sept to Nov 2011) , spring wheat planting (Apr 2011) and harvest (July
2012), shown in Table 4.8........................................................................................................... 171
xii
Table D.16 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat yield
or biomass following the early and late broccoli systems with 15N enriched residual fertilizer or
broccoli root-derived N, shown in Table 4.9. ............................................................................. 173
Table D.17 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat N
derived from 15N enriched residual fertilizer or broccoli root following the early and late broccoli
systems, shown in Table 4.9. ...................................................................................................... 173
Table D.18 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat total N
(14N and 15N) after early and late broccoli systems with 15N enriched residual fertilizer or
broccoli roots, shown in Table 4.9. ............................................................................................. 174
xiii
LIST OF FIGURES
Figure 1.1 The nitrogen cycle. ........................................................................................................ 3
Figure 1.2 Typical interaction of soil mineral N concentration and microbial activity over time
after material with a low C:N ratio (<25) is added to soil. ............................................................. 7
Figure 1.3 The typical interaction of soil mineral N concentration and microbial activity over
time after material with a high C:N ratio (>25) is added to soil. .................................................... 9
Figure 1.4 The average annual water budget in southwestern Ontario, as estimated by Fallow et
al. (2003). ...................................................................................................................................... 12
Figure 2.1 Net N mineralization (mg N kg-1dry soil) as affected by organic C amendment with
broccoli residue-derived N (left) or fertilizer-derived N (right), during the 56-d incubation. Data
points represent the mean of observed values (n4) and bars represent the standard error of the
mean but may be smaller than the symbols. ................................................................................. 41
Figure 2.2 Net C mineralization (mg C kg-1 dry soil) as affected by organic carbon amendment
with broccoli residue- derived N (left) or fertilizer-derived N (right), during the 56-d incubation.
Data points represent the mean of observed values (n4) and bars represent the standard error of
the mean but may be smaller than the symbols. ........................................................................... 47
Figure 3.1 Soil mineral N concentrations in the autumn, spring, and summer after the 2009 and
2010 early- and late-broccoli harvest treatments. Symbols denote a significant difference (P <
0.05) compared to the crop residue control * or the crop residue with pre-plant N control +,
based on a multiple means comparison with a Dunnett-Hsu adjustment. The se values represent
the standard error of the mean. ..................................................................................................... 65
Figure 3.2 The N content (kg N ha−1) of spring wheat plant biomass (grain + straw) and grain in
2010 and 2011, as affected by the previous years’ treatments in the early- and late-broccoli
systems. Symbols denote a significant difference (P < 0.05) compared to the crop residue control
* or the crop residue with pre-plant N control +, based on a multiple means comparison with a
Dunnett-Hsu adjustment. The se values represent the standard error of the mean. ...................... 72
Figure 3.3 The spring wheat plant biomass (grain + straw) and grain yield (Mg ha−1) in 2010 and
2011, as affected by the previous years’ treatments in the early- and late-broccoli systems.
Symbols denote a significant difference (P < 0.05) compared to the crop residue control * or the
crop residue with pre-plant N control +, based on a multiple means comparison with a DunnettHsu adjustment. The se values represent the standard error of the mean. .................................... 73
xiv
Figure 4.1The effect of used cooking oil on 0-60 cm soil A) total, B) mineral, and C) organic Nderived from crop residue (kg ha-1) expressed as a percentage (%) of the control over time
(months). Bars represent the experimental means with standard error and the solid line represents
the prediction according to the first order exponential model y =A(1-exp-k(time)), (y=percent of
control, A=coefficient indicates potential N, k=rate constant, t=time). ..................................... 103
Figure A.1 In 2009, the effect of alternative post-harvest strategies on soil nitrate concentrations
after early harvested broccoli (4 Aug). Based on the last sample day, *asterik denotes a statistical
difference between treatment and incorporated crop residue (control). ..................................... 138
Figure A.2 In 2010, the effect of alternative post-harvest strategies on soil nitrate concentrations
after early harvested broccoli (3 Aug). Based on the last sample day, *asterik denotes a statistical
difference between treatment and incorporated crop residue (control). ..................................... 138
Figure A.3 In 2009, the effect of alternative post-harvest strategies on soil nitrate concentrations
after late harvested broccoli (31 Aug). Based on the last sample day, all treatments were not
different from the incorporated crop residue (control). .............................................................. 139
Figure A.4 In 2010, the effect of alternative post-harvest strategies on soil nitrate concentrations
after late harvested broccoli (22 Sept). Based on the last sample day, *asterik denotes a statistical
difference between treatment and incorporated crop residue (control). ..................................... 139
Figure A.5 Effect of alternative post- broccoli harvest strategies on mean spring wheat grain
yield in 2010 and 2011. * Asterik denotes a statistical difference between treatment and
incorporated crop residue (control). ............................................................................................ 140
Figure A.6 Effect of alternative post- broccoli harvest strategies on mean spring wheat grain N
content in 2010 and 2011. * Asterik denotes a statistical difference between treatment(*) and
incorporated crop residue (control). ............................................................................................ 140
Figure B.1 Net C mineralization (mg C kg-1 dry soil) as affected by organic carbon amendment
with broccoli residue- derived N (left) or fertilizer-derived N (right), during the 56-d incubation.
Data points represent the mean of observed values (n=4) and bars represent the standard error of
the mean but may be smaller than the symbols. Refer to section 2.3.1 and 2.3.2 for calculation of
net mineralization........................................................................................................................ 151
xv
ABBREVIATIONS
BMP: best management practice
CR-control: crop residue incorporated control
CRN: broccoli above-ground crop residue-derived N, based on 15N tracer
CRN-control: crop residue incorporated with N fertilizer applied to spring wheat
CR-removal: crop residue removed
FN: fertilizer-derived N, based on 15N tracer
NUE: nitrogen use efficiency
OCA: organic carbon amendment
OCA-oil: organic carbon amendment of used cooking oil
OCA-straw: organic carbon amendment of used cooking oil
OCA-yard: organic carbon amendment of used cooking oil
RN: residual fertilizer-derived or broccoli root biomass-derived N, based on 15N tracer
SMN: soil mineral N
TN: total stable isotope N (14N + 15N)
1
INTRODUCTION
1.1 NITROGEN CYCLE
Nitrogen is vital for life on earth as it is integral to many biological components, such as
nucleic acids, amino acids, and proteins. Yet, despite its abundance as nitrogen gas (N2) in the
atmosphere (78 %) it is inert and unavailable to most living organisms. Three methods of
xvi
converting N2 into biologically available forms include biological N fixation, lightning fixation,
and industrial fixation. Globally, natural processes of biological and lightning fixation was
estimated to produce 112 Tg annually until 1960s in terrestrial ecosystems, but anthropogenic
(both industrial and biological N2 fixation) now fixes over 187 Tg of N per year (Galloway et al.
2004; Galloway et al. 2008). It has been estimated that the availability of N has increased by
120% during the last century (Galloway et al. 2008). Notwithstanding the importance to life, N
can negatively impact the environment depending on the chemical form, abundance, and
location. The following sections address the chemical forms of N, the cyclical movement of N
(Figure 1.1), and highlights agricultural practices for nutrient management.
2
Figure 1.1 The agricultural nitrogen cycle.
1.1.1
Nitrogen Fixation
The biological N fixation process is conducted by certain symbiotic and non-symbiotic
bacteria and some archaea. All N fixing species possess the central enzyme nitrogenase to
catalyze the reaction [equation 1].
N2 + 8e- + 8H+ + 16ATP  2NH3 + H2
[1]
The symbiotic relationship involves an association between a prokaryote (ie: Rhizobium,
Klebsiella, Nostoc, Frankia) and a eukaryote plant host, while non-symbiotic prokaryotes live
3
freely. Biological N fixation requires 16 ATP and produces NH3 which is subsequently
combined with organic acids to form amino acids and proteins. By incorporating leguminous (N2
fixing) crops into rotations, growers can increase soil N fertility for subsequent crops.
Alternatively, lightning can provide enough energy to fix N according to the successive
reactions [equations 2 to 4] in the atmosphere. The product HNO3 is nitric acid, which can enter
terrestrial or aquatic ecosystems via wet or dry deposition.
N2 + O2 + lightning  2NO
[2]
2NO + O2  2NO2
[3]
2NO2 + H2O  HNO3 + HNO2
[4]
Industrial N fixation occurs via the Haber-Bosh process, catalyzed under high pressure
and temperature to produce NH3 [equation 5]. Additional chemical reactions produce the desired
N compound, such as urea (NH2)2CO under high pressure, [equation 6], HNO3 under high
temperature [equation 7], and ammonium nitrate (NH4NO3), [equation 8].
N2 + H2  NH3
[5]
NH3 + CO2  (NH2)2CO
[6]
NH3 + O2  HNO3
[7]
NH3 + HNO3  NH4NO3
[8]
The products of industrial fixation can be used for agricultural fertilizers and explosives.
Industrially produced N fertilizer can significantly increase crop yields. The dependence on N
fertilizer is so great that a large portion of the human population may be sustained on the use of
synthetic fertilizer (Smil 1999; Galloway et al. 2003). The amount of synthetic fertilizer required
4
for crop production must be calculated after potential N sources are taken into account (organic
matter, soil available N). Appropriate quantity, timing, type, and method of fertilizer application
are critical for crop production, but are also important for preventing excess soil N and losses.
1.1.2
Nitrogen Mineralization
The N mineralization process is carried out in soil by heterotrophic microbes [equations 9
and 10].
Protein compound  R-NH2 + CO2
[9]
R-NH2 + 2H2O  R-OH + OH- + NH4+
[10]
Mineralization is a function of decomposition and is largely influenced by soil
temperature, moisture, aeration, pH, as well as the composition of the decomposing organic
matter. Mineralization proceeds optimally at temperatures of 30-40°C, pH of 6-8, and soil water
filled pore space of 60% (Myrold and Bottomley 2008). The C:N ratio and chemical composition
of the decomposing matter influences decomposition rates, with soluble compounds (i.e. simple
proteins, starches, sugars) being more rapidly decomposed than complex compounds (i.e. lignin).
Quantitatively, the most important N containing molecules are chitin, protein, and peptidoglycan.
Extracellular depolymerases break down N containing molecules to smaller organic molecules,
which may be directly taken up by microbes or ammonified to NH4+ and nitrified to NO3-.
Microbial enzymes involved in the decomposition are included below (Table 1.1).
5
Table 1.1 Types of microbial enzymes involved in decomposition. *
Substrate
Microbial Enzyme
Function
Proteins
Proteinases
Cleaves large proteins
Proteins
Peptidases
Cleaves tri- or di-peptides or split off individual
amino acid
Nucleic Acids
Chitin
Ribonucleases and
Hydrolyzes ester bonds between phosphate groups
Deoxyribonucleases
and pentose sugars
Chitinase or
Breaks down chitin into dimers of chitobiose, then
chitobiase
into N-acetylglucosamine and Nacetylglucosaminidase
Bacterial Cell
Lysozyme
Breaks down peptidoglycan part of bacterial cell wall
Wall
into final product of individual amino sugars, which
Amino Sugars
is phosphorylated and deaminated to release NH4+
Urea
Urease
Hydrolyzes urea into CO2 and NH3
* Information derived from Myrold and Bottomley (2008).
Generally, if the substrate has a C:N ratio < 25 the microbial requirement for N may be
less than the quantity of N contained in the decomposing matter and excess N is released as
NH4+ (Figure 1.2). Otherwise if the substrate has a C:N ratio > 25, the N requirement may be
greater than the N supplied and additional N from the surrounding soil is immobilized into soil
microbial biomass (Figure 1.3), (Cabrera et al. 2005; Myrold and Bottomley 2008).
6
Figure 1.2 Typical interaction of soil mineral N concentration and microbial activity over time after
material with a low C:N ratio (<25) is added to soil.
1.1.3
Nitrogen Immobilization
During decomposition, heterotrophic microbes assimilate available N [equation 11].
NH4+ or NO3- or organic N  microbial cells + CO2
[11]
This process can occur via the mineralization-immobilization turnover route or the direct
route. The mineralization-immobilization route ensues as mineralized N is subsequently
immobilized, while the direct route occurs as N in small organic compounds is directly
incorporated into microbial cells.
7
Immobilization is governed by the same aforementioned factors as N mineralization.
Shortly after the addition of high C material (C:N ratio >25.1) to soil, heterotrophic flora become
active, multiply rapidly and release carbon dioxide via respiration (Figure 1.3). As a result,
available soil N concentration is lowered due to high microbial demand and less N is available to
plants (Figure 1.3). During decomposition, recalcitrant organic compounds may persist and the
substrate’s C:N ratio may lower due to microbial C consumption, which could eventually favor
re-mineralization (Figure 1.3). When the readily oxidizable C fraction is consumed, the
microbial population declines and N ceases to be in-demand, allowing nitrification to proceed
(Brady 1974). Over a longer-term however, lignin and its breakdown products can condense with
organic N to form highly stable organic N thereby reducing net mineralization (Myrold and
Bottomley 2008).
8
Figure 1.3 The typical interaction of soil mineral N concentration and microbial activity over time
after material with a high C:N ratio (>25) is added to soil.
1.1.4
Nitrification
Subsequent to mineralization or the addition of fertilizer/manure, nitrification proceeds
via a two-step process converting NH4+ to NO3-. The first step [equation 12], is typically
facilitated by chemo-autotrophic bacteria in the genera Nitrosomonas or Nitrosococcus or
Nitrosospira which supply the catalyst ammonium-oxidase. The second step [equation 13] is
generally facilitated by bacteria in the genera Nitrobacter or Nitrococcus or Nitrospira, which
supplies the catalyst nitro-oxioreductase. Some hetertrophic nitrifiers exist, such as Aspergillus
fungi, and Acaligenes bacteria.
9
NH4+ + 1½O2  NO2- + 2H+ + H2O
[12]
NO2- + ½O2  NO3-
[13]
Nitrification proceeds optimally at temperature 30-35°C, soil pH 6.6-8, and soil water
filled pore space of 60% (Norton 2008). Low soil pH (<5) can slow or inhibit nitrification, and
high soil pH can slow the second step of nitrification, thus permitting the accumulation of nitrite
(NO2-) which may cause a toxic environment for plants and other organisms (Norton 2008).
The rate of nitrification of fertilizer-derived N can be managed with slow-release
fertilizers or nitrification/urease inhibitors. Controlled-release fertilizers have coatings (i.e.
polymer or S) which must decompose prior to the release of NH4+ and nitrification; thereby
slowing the formation of NO3-. Nitrification inhibitors temporarily disable Nitrosomas, thus
slowing the first step of nitrification [equation 12]. Ideally, the use of controlled-release
fertilizers or nitrification inhibitors may produce available N closer to the period of rapid plant
crop growth, otherwise, pre-plant N fertilizer may be at risk of loss during a wet spring prior to
crop N uptake. However in practice, slow-release fertilizers may not match crop N demand in
southwestern Ontario (Van Eerd 2010) and have not shown any agronomic advantage over urea
when applied in the fall (Beauchamp 1977).
1.1.5
Nitrate Leaching
Nitrate is mobile in soil, due to its size and negative charge. It readily solubilizes in water
and is transported in soil via mass water flow. Nitrate moves downward with percolating water
through the vadose zone and into groundwater, a process called nitrate leaching. Subsurface
drainage tiles are common in Ontario to drain excess water within the vadose zone for optimal
10
cropping production; however drainage tiles may increase preferential flow and therefore sorbed
NH4+ transport to surface waters. The risk of leaching is positively related to soil NO3concentration, soil permeability, water content, water flow, and temperature. In agricultural
systems where excess NO3- is available, there may be a high risk of NO3- leaching during
autumn, winter, and early spring in Ontario (and other areas with similar climates) due to the
annual water budget (Fallow et al. 2003) (Figure 1.4). Once NO3- contaminates groundwater, it
may further contaminate creeks, rivers, ponds, lakes, etc. via the hydrologic cycle. An elevated
NO3- concentration in water bodies may have negative impacts, such as drinking water
contamination or eutrophication in salt water systems. Furthermore, NO3- in surface waters may
be subsequently denitrified. Nitrate leaching can be managed by matching crop N demands to
NO3- availability, and/or by reducing NO3- availability during non-cropping seasons, as discussed
later.
11
180
Precipitation
160
Evapo-transpiration
140
Water (mm)
120
100
80
60
40
20
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sept
Oct
Nov
Dec
Figure 1.4 The average annual water budget in southwestern Ontario, as estimated by Fallow et al.
(2003).
1.1.6
Surface Runoff
Dissolved NO3- (and NH4+) and adsorbed NH4+ on soil particles can be transported with
water as it moves horizontally along a soil surface, a process known as surface runoff and
erosion. Surface runoff occurs when the infiltration capacity of the soil is exceeded and water
flows over land. Methods to reduce the volume of surface runoff and N losses from agricultural
systems include cover crops and leaving crop residues on the soil surface (particularly during
non-cropping seasons).
12
1.1.7
Denitrification
Nitrogen can be returned to the atmosphere via the reduction of NO3- to N2 [equation 14].
2NO3- + 10e- + 12H+ N2 + 6H2O
[14]
The stepwise conversion process involves 2NO3- to 2NO2- to 2NO to N2O to N2 and is
carried out primarily by heterotrophic anaerobic bacteria species such as Acaligenes faecalis,
Paracoccus denitrificans, and bacteria in the genera Bacillus and Pseudomonas, as well as some
autotrophic bacteria species, Thiobacillus denitrificans. The enzymes involved in the stepwise
conversion are nitrate reductase, nitrite reductase, nitric oxide reductase, and nitrous oxide
reductase, respectively. Denitrification is largely governed by low soil aeration, high moisture,
and high NO3- and C availability (Myrold and Bottomley 2008). Denitrification is therefore
likely to proceed in wet soils, wetlands, and water which have NO3- and C available. Other
factors such as soil pH and temperature also influence denitrification, and soil pH 6-8 and
temperatures in the 5-75°C range are conducive to denitrification. Prior to the completion of the
stepwise reduction [equation 14], gaseous N oxides (NOx) or N2O may escape into the
atmosphere if conditions are temporarily or partially anaerobic. Consequences of such N losses
are discussed later, such as ground-level ozone (O3) production and atmospheric radiative
forcing.
In agricultural systems, strategies to reduce soil N losses via denitrification include
synchronizing soil NO3- availability with crop demand and reducing excess soil NO3- during noncropping seasons. Such practices include realistic yield goals (not over applying N fertilizer),
placing fertilizer deep in the soil profile and incorporating crop residues to minimize diffusion of
13
gaseous products of denitrification, and by establishing cover crops or tile drainage to reduce
saturated anaerobic soil conditions.
1.1.8
Ammonia Volatilization and Ammonium Adsorption
Ammonium ions are in equilibrium with NH3, described by the reversible reaction
[equation 15]. If the reaction [equation 15] proceeds to the right, NH3 gas can be released to the
atmosphere.
NH4+ (aq) + OH- (aq) ↔ NH3 (g) + H2O (l)
[15]
The process is governed by pH, water availability, temperature and the equilibrium of
dissolved NH3 gas in soil with the NH3 gas in the air (NH3 (g) ↔ NH3 (atm)). A high soil pH
environment will result in production of NH3 as the reaction is driven by OH-. A concentrated
release of NH3 is toxic. Ammonium cations adsorb to negatively charged clay and organic matter
particles based on the cation exchange capacity of the soil, thus adsorbed NH4+ is in equilibrium
with NH4+ in solution. Adsorbed NH4+ can be subjected to loss via soil erosion. Surface placed
amendments, such as urea or manure, are at a high risk for gaseous N loss via NH3 volatilization.
Thus, incorporating manure and urea into soil will reduce the risk of volatilization by reducing
diffusion from soil into the atmosphere.
1.1.9
Plant Uptake
Nitrogen is available to plants in the forms of NH4+, NO3- and small soluble organic N
molecules (i.e. amino acids). The nutrients move to the root surface via mass flow and diffusion.
Mass flow results in the passive uptake of soluble N (i.e. NO3-) by plants because the nutrients
14
dissolved in soil water flow to roots and shoots via the potential water gradient caused by
transpiration. Ammonium moves passively to the plant roots from the bulk soil when the
concentration difference causes a diffusion gradient towards roots. The acquisition process is
related to the chemical form of N, active uptake, and/or biological N fixation. Active NO3- and
NH4+ uptake in roots is summarized by the following reactions, catalyzed by nitrate reductase
[equation 16], nitrite reductase [equation 17], glutamine synthetase [equation 18], and glutamate
synthase [equation 19], respectively. At concentrations similar to that of soil inorganic N, soluble
dissolved organic N is often available (due to rapid hydrolysis of soil proteins) and may be taken
up by plants via amino acid transporters, as reviewed by Näsholm et al. (2009). However the
uptake of organic N may be largely a function of the recapture of amino acids exuded by roots
(Jones et al. 2005).
NO3- + 2e-  NO2-
[16]
NO2- + 6e-  NH4+
[17]
NH4+ + glutamate + ATP  glutamine + ADP + Pi
[18]
glutamine + α-ketoglutarate + 2e-  glutamate
[19]
The transport of N containing compounds (i.e. NO3- and amino acids) in the phloem is
controlled by loading pressure of carbohydrates via photosynthesis at C sources (i.e. leaves)
causing the flow and transport to C sinks (i.e. roots, newly developing shoots, and/or
reproductive organs). Resorption of amino acids in older leaves prior to senescence is important
for the re-translocation of N to support new growth and development of expanding leaves or
fruits (Peoples and Gifford 1997). Partitioning of N between roots, vegetative and reproductive
tissue depends on environmental factors or nutritional status such as soil N concentration. In
15
agriculture, optimizing soil N fertility is essential for plant growth, development, and production
and is discussed later in the context of crop production.
1.2 NITROGEN LOSSES
1.2.1
Consequences of Nitrogen Losses
Nitrate leaching, surface runoff, denitrification, and ammonia volatilization can result in
N losses from agricultural systems. Globally, the largest source of groundwater NO3contamination was from agricultural ecosystems, via nitrate leaching (Galloway et al. 2003).
Once in groundwater the fate of NO3- is either accumulation, conversion via the denitrification
pathway, or distribution along hydrologic pathways (Galloway et al. 2003).
Accumulation of NO3- in soil or groundwater is not considered a major sink due to the
high mobility of NO3-; however elevated levels of NO3- in groundwater may pose a health
concern, if consumed. The following reaction [equation 20] may ensue in the stomach after the
consumption of N contaminated water, which may result in methemoglobinemia.
NO3- + 2H+  NO2- + H2O
[20]
Methemoglobinemia is a condition in which NO2- binds to hemoglobin instead of O2 and
causes suffocation (blue baby syndrome). Due to the presence of NO3- reducing bacteria in
infants’ stomachs, infants are more susceptible than adults to methemoglobinemia. Ruminants
are susceptible to methemoglobinemia as well. Other conditions such as respiratory infections,
changes in thyroid metabolism, and cancer may develop after consuming NO3- contaminated
drinking water (Galloway 2003). The safe and recommended limit of NO3- in drinking water
according to the World Health Organization has been established as 10 mg NO3--N L-1.
16
Nitrate contamination in ground or surface water can have negative environmental
effects. Distribution of NO3- along hydrologic paths can result in eutrophication downstream.
Eutrophication is characterized by phytoplankton blooms due to the addition of nutrients to a
water body. Although both N and P contribute to eutrophication, the addition of the key limiting
nutrient regulates eutrophication. For example, fresh vs. salt water eutrophication is typically due
to P vs. N contamination, respectively. The nutrient availability causes a rapid increase in
phytoplankton biomass which is subsequently decomposed by bacteria, a process which
consumes O2. Anoxic conditions can develop which can negatively impact aquatic species.
The conversion of NO3- via denitrification can return N2 to the atmosphere; however the
production of greenhouse gases (NOx or N2O) may also result. The atmospheric radiative forcing
of NOx or N2O may contribute to climate change (IPCC 2007). Atmospheric contamination of
gaseous NOx and/or NH3 can result in decreased atmospheric visibility (fine particulate matter),
increased production of ground-level O3, serious impacts on human health (from particulate
matter and O3), aerosol radiative forcing from NH3, and decreased productivity of natural,
agricultural, and forest ecosystems (Galloway et al. 2003). Thus, groundwater and atmospheric N
contamination are significant concerns and research must focus on BMPs that reduce
environmental N contamination.
1.2.2
Seasonal Risks of Soil Nitrogen Losses
While the soil is frozen in winter in Ontario, microbial activity and water movement is
limited and there is little risk of NO3- leaching or denitrification. However, during mild winter
periods and spring, the soil water content is increased due to snow or soil thawing and
precipitation, and the risk of downward water movement is high. As soils warm in spring, soil
17
microbial activity and soil mineral N (SMN) concentrations increase. In agricultural systems,
often fertilizer or manure is applied which may contain NH4+, NO3-, both, and/or urea. Urea is
hydrolyzed to NH4+, and is rapidly nitrified by Nitrobacter.
During May and June, when N is typically applied, crops have not yet entered a period of
active growth and may take up little soil mineral N. The soil water content may increase and
water may flow downward, due to greater precipitation than evapotranspiration. The
combination of applied N fertilizer, active soil microbes (for mineralization, nitrification, and
denitrification) and soil water content during springtime results in a period of high risk of N
losses (leaching, denitrification, or surface runoff).
During Ontario summer months the plants/crops typically enter a period of rapid growth
and N uptake. Evapotranspiration typically exceeds precipitation (Figure 1.4), thus, there is little
risk for N losses (leaching, denitrification, or surface runoff) during summer months.
However in autumn and after crops are harvested, precipitation usually exceeds
evaporation (Figure 1.4), the soil water content may increase and water may flow downward.
Agricultural fields are generally void of crops and there is little opportunity for mineral N to be
taken up from the soil. Therefore, autumn is a period of high risk for leaching, denitrification, or
surface runoff due to availability of soil mineral N and high soil water content.
Nitrogen losses are therefore regionally related to the water budget. According to the
water budget estimated by Fallow et al. (2003) (Figure 1.4), there is a high risk of losses during
the autumn, mild winter periods, and spring in southwestern Ontario (and elsewhere with similar
climates).
18
1.3 COLE CROP PRODUCTION AND NITROGEN MANAGEMENT
1.3.1
General Cole Crop Production in Ontario
Cole crops are Brassicaceae vegetables which are frost tolerant and can be produced in
temperate zones such as Ontario. Cole crops including broccoli, Brussels sprouts, cabbage, and
cauliflower (Brassica oleracea L.) have been produced on up to 4663 hectares in Ontario during
recent years, according to the Ontario Ministry of Agriculture, Food, and Rural Affairs
(OMAFRA 2012a, 2013a). The high nutritional and monetary value of cole crops (i.e. up to $
1722.9 Mg-1 (OMAFRA 2013a)) may lead to expanded production on southern Ontario soils,
most of which are Luvisolic, Gleysolic, or Brunisolic.
Soils with neutral to slightly acidic pH (5.5 to 7.6) and have good structure, fertility, and
water-holding capacity are preferable for producing cole crops (OMAFRA 2004). Cole crops can
be successfully produced on most soil textures (OMAFRA 2004) and sandy loam or clay loam
soils are common for Ontario cole crop producers. Ample soil moisture is essential for successful
cole crop production and growers usually irrigate when necessary (OMAFRA 2004).
Cole crops such as broccoli are typically grown from transplants which are planted in the
field from late April to mid-August, with spacing of approximately 40 by 60 cm (OMAFRA
2013b). Within 70 d from transplanting the broccoli crop will be ready for harvest, thus harvest
may occur anytime from July to November in Ontario (OMAFRA 2013b). Successional broccoli
planting from spring to summer can ensure regular harvests and market supply throughout the
summer and fall. The main terminal heads are harvested prior to flowering. Mechanical harvest
aids can be used; however hand-harvest is more common in Ontario (OMAFRA 2013b). The
19
crop residue (i.e. stem and leaves) which remains in the field is commonly mulched and
incorporated into the soil.
Considering the wide harvest window for cole crop production in Ontario, cole crop
research must account for early and late production systems because environmental conditions
(i.e. temperature and moisture) vary between early harvest (i.e. July or August) and later harvest
(i.e. September to November).
1.3.2
Nitrogen Use Efficiency
To produce cole crops, growers typically apply large amounts of N fertilizer at planting
(i.e. >300 kg N ha-1) to increase yield and profits (Zebarth et al. 1995; Bakker et al. 2009a),
despite lower N fertilizer recommendations (i.e. 130 kg N ha-1 for Ontario (OMAFRA 2006)).
For instance, marketable broccoli yield increased to a maximum between N rates of 435 and 560
kg N ha-1, while profit was maximized with 395 to 508 kg N ha-1 (Zebarth et al. 1995). Bakker et
al. (2009a) found N fertilizer rates of 298 to 309 kg N ha-1 were most economical for broccoli
production. Thus, N fertilizer is often liberally applied for cole crop production to ensure
agronomic and economic profitability. Such high N fertilizer application can pose a significant
threat for environmental contamination, via N losses (i.e. ammonia volatilization, denitrification,
leaching, or surface runoff). As society or policy demands more environmental accountability
from farmers, growers may be motivated (socially, economically, or by regulation) to adopt
management practices which minimize nutrient losses (Beegle et al. 2000).
It is therefore necessary to determine the best N management strategies which improve
the utilization efficiency of N fertilizer and also mitigate potential N losses for cole crop
20
production. Best N management practices can be grouped into two broad categories: (i)
improving the N use efficiency (NUE) (such as optimizing N fertilizer supply, rates, placement,
timing) and (ii) reducing N losses from the cropping system (such as reducing the quantity or
mobility of N after harvest) (Zebarth et al. 2009).
Maximizing crop NUE is an important strategy to improve the fertilizer use in
agricultural systems. With fertilizer applications of 500 to 625 kg N ha-1, apparent fertilizer N
recoveries (i.e. regression of plant N content at harvest by soil N fertilizer applied) for cole crops
ranged from 20 to 44% (Zebarth et al. 1995; 1991). Also, with fertilizer applications of 100 to
840 kg N ha-1, 15N tracer studies have found fertilizer recoveries of 16 to 38% in above-ground
cabbage crop tissue (Choi et al. 2004; Nissen et al. 1999). Thus, the majority (56 to 84%) of
fertilizer N may be susceptible to losses and cole crop growers should adopt practices which
improve crop fertilizer N use. Research has shown that NUE increased as N fertilization rates
decreased (Zebarth et al. 1995; Bakker et al. 2009b). However, lowering N fertilizer rates could
also lower cole crop yields (Bakker et al 2009a; 2009b). Thus, improving cole crop NUE by
reducing N fertilizer applications is unlikely to be adopted by growers and alternative practices
which do not risk lowering cole crop yields must be developed.
Strategies which reduce N losses after cole crop production may be more essential than
in-season N management. After cole crop harvest, there is a high risk for N losses and possible
environmental contamination. Residual N after harvest (soil mineral and crop residue N) may
range from ≈ 100 to 415 kg N ha-1 (Thompson et al. 2002; Bakker et al. 2009b). Due to the
annual water budget (Fallow et al. 2003) and the high levels of residual N, there is a great risk
for N loss during post-harvest season. Zebarth et al. (1995) found SMN concentrations of 100 kg
21
ha-1 in the top 75 cm profile at broccoli harvest. Thus, the residual N derived from indigenous
soil mineralization, below-ground crop residue, and/or fertilizer could be susceptible to losses
after harvest. However, the above-ground crop residue had over twice the amount of N as the 75
cm soil did by harvest (Zebarth et al. 1995). Although much research has been done on the
development of techniques to reduce N losses during the cropping season (Zebarth et al. 1995;
Thompson et al. 2003; Bakker et al. 2009a), research on minimizing N losses during the postharvest season is lacking despite its necessity.
1.3.3
Cover Crops
One method of minimizing post-harvest N losses is the establishment and growth of
cover crops. Cover crops take up soil mineral N thereby lowering the quantity of N available for
leaching or denitrification, and ideally plant biomass decomposition corresponds to the release of
available N for subsequent crop uptake. Also, cover crop transpiration lowers soil moisture, and
roots may increase water infiltration, lower runoff, and reduce surface or groundwater N
pollution (Hartwig and Ammon 2002).
However, cole crops can be harvested as late as November in Ontario which leaves little
time for cover crop establishment. In order to overcome this limitation, an undersown cover crop
is suggested (Everaarts 1993), however there must be enough time for cover crop establishment,
growth, and biomass production, yet minimal competition for the harvested crop and low
interference with harvest operation. An alternative option for minimizing N losses is the removal
of broccoli crop residue from the field (Everaarts 1993).
1.3.4
Crop Residue Management
22
The management of crop residues also influences N cycling. Crop residues on the soil
surface may minimize overwinter NO3- leaching losses (Wehrmann and Scharpf 1989). Leaving
crop residues on the soil surface may require less labour or time, reduce soil erosion, and
increase available water and fungal growth compared to incorporated crop residues (Smith and
Sharpley 1990). However, residues on the soil surface may decompose more slowly than
incorporated crop residues because the incorporated residues are in closer proximity to soil C
(Smith and Sharpley 1990) and soil microbes, and the soil water and temperatures are more
consistent at depth than on the surface. However, once the surface crop residues decompose,
there could be a greater risk of surface runoff N losses compared to incorporated residues.
Thus, it is important to characterize the decomposition of crop residues, such as cole crop
stems and leaves. De Neve and Hofman (1996) found that a first order kinetic model described N
mineralization of incorporated cole crop residues: N(t)=NA (1- e-kt), where N(t) is the percentage
of total residue N released at time t, NA is the amount of mineralizable N expressed as a
percentage of total residue N, k is the rate constant, and t is the time from the start of the
incubation. Decomposition of cole crop leaves, upper stem, and lower stem demonstrated rapid
N mineralization during the first three weeks after crop residue incorporation, although
mineralization was greater for the upper stem and leaves released than the lower stem (De Neve
and Hofman 1996). However, growers do not typically separate cole crop residues into leaves,
upper and lower stems. The traditional post-harvest strategy for broccoli crop production is to
mulch and incorporate cole crop leaves and stems into the soil following harvest. In order to
accurately assess the potential of post-harvest strategies to reduce N losses, it is essential to
characterize N mineralization rates from a homogenous mixture of cole crop stem and leaves.
23
1.4 ORGANIC CARBON AMENDMENTS
Another management strategy to prevent N losses could be amending the soil with a
readily decomposable C rich substrate (C:N >25) to stimulate biological N immobilization after
crop harvest in the fall. Hypothetically, by applying C rich amendments and if the conditions and
timing are ideal, N losses may be reduced due to immobilization throughout the fall and winter
and N may be re-mineralized for subsequent crop uptake.
Soil organic matter and nutrient cycling are affected by organic C amendments (OCA)
(Jawson and Elliot 1986; Mary et al. 1996; Plante and Voroney 1998; Bending and Turner 1999;
Amlinger et al. 2003; Rashid and Voroney 2003). Nitrogen immobilization has reduced the
potential for fertilizer- or soil-derived N losses after the application of OCA such as oily food
waste (Plante and Voroney 1998; Rashid and Voroney 2003; Rashid and Voroney 2004) wheat
straw (Jawson and Elliot 1986; Mary et al. 1996; Bending and Turner 1999) and yard waste
(Hartz and Giannini 1998; Amlinger et al., 2003). However, most studies have investigated N
dynamics with OCA and applied N fertilizer (Plante and Voroney 1998; Goyal et al. 1999,
Rashid and Voroney 2003; Yadvinder-Singh et al. 2004) and not with crop residue-N.
Previously, it has been difficult to match OCA decomposition rates with that of vegetable crop
residues (i.e. high N residues of pea and groundnut) (Yadvinder-Singh et al. 2004; Kaewpradit et
al. 2008). For the development of effective BMPs which minimize N losses, future research must
evaluate the synchrony of cole crop residue and OCA decomposition rates.
It is also necessary to assess potential effects of OCA on the subsequent crop. While
some amendments may result in reduced N losses via N immobilization, certain amendments
may accumulate and cause soil hydrophobicity. Fatty acid application concentrations of more
24
than 400 mg kg-1 soil have been documented to cause water repellency (Ma’shum et al. 1988).
However, other studies have found no evidence of water repellency after lipid applications (Bond
1968) and very little accumulation one year after application of fat-oil-grease (Rashid and
Voroney 2004). Prolonged N immobilization may reduce N availability to subsequent crops
(Bremer and van Kessel 1992; Mary et al. 1996; Yadvinder-Singh et al. 2004; Singh et al. 2006).
If N immobilization due to the autumn application of OCA is not followed by re-mineralization
during subsequent plant uptake, a negative effect on yield could result. A pattern of fall N
immobilization followed by subsequent spring/summer N re-mineralization was observed in
some OCA studies (Rashid and Voroney 2003; Rashid and Voroney 2004), yet others have
found no re-mineralization after a year (Mary et al. 1996; Chaves et al. 2007a). Alternatively, if
N re-mineralization is synchronous with the subsequent crop N requirement, N use efficiency
could be enhanced.
Careful selection of the type of OCA is crucial for the development of an effective BMP
to minimize N losses. In addition to the influence on decomposition, other factors such as OCA
availability, effect on subsequent crop, and cost-benefit should be considered to select the best
OCA to reduce N losses after cole crop production.
1.5 CONCLUSIONS
Despite the vital importance of N to living organisms, there are deleterious effects from
N losses such as ground or surface water contamination, atmospheric radiative forcing, and
ground-level ozone production. Therefore, N losses must be kept low to mitigate environmental
degradation of our hydrosphere and atmosphere. Novel and effective management strategies are
needed to minimize N losses after cole crop harvest due to the large quantities of readily
25
mineralizable N in crop residues and the high risk for N leaching, denitrification, or surface
runoff.
1.6 RESEARCH OBJECTIVES
This research investigates the potential for minimizing N losses by implementing a novel,
innovative practice of applying OCA after cole crop harvest. It is hypothesized that OCA will
immobilize crop residue-derived N and thereby reduce potential soil N losses after cole crop
harvest. The objectives were to:
(i) Characterize the effects of three organic C amendments (wheat straw, yard waste, and
used cooking oil) on N mineralization-immobilization dynamics and microbial activity
under controlled conditions in vivo, when the source of N was derived from broccoli
residue or fertilizer.
(ii) Evaluate the effects of three organic C amendments (wheat straw, yard waste, and used
cooking oil) on SMN concentration, spring wheat N content, yield, profit margins
following broccoli harvest, compared to typical grower practices in situ.
(iii) Assess of the fate of above-ground and below-ground broccoli residue-derived N using a
15
N tracer technique in soil N pools (total, mineral, microbial, organic) after harvest and
into subsequent spring wheat production, with and without the amendment of used
cooking oil.
The potential for reducing N losses after cole crop harvest were assessed with treatment
comparisons of (a) crop residue incorporation (traditional practice) to (b) crop residue
26
incorporation with the C amendments (novel practice) (Congreves et al. 2013a; 2013b;
Congreves et al. 2014). The experiments included:
(i) a two month microcosm study under controlled conditions in the laboratory (Chapter 2).
(ii) a repeated two year field study (2009-2010 and 2010-2011) from early (August) and late
(September) broccoli harvest systems to subsequent spring wheat production (Chapter 3)
(iii) a two year (2011-2012) micro-plot field study using isotopic 15N techniques to trace crop
residue-derived N movement from early (August) and late (September) broccoli harvest
systems to subsequent spring wheat production (Chapter 4).
27
2
BROCCOLI RESIDUE-DERIVED NITROGEN IMMOBILIZATION FOLLOWING
AMENDMENTS OF ORGANIC CARBON: AN INCUBATION STUDY.
2.1 ABSTRACT
Cole crops, compared to many other crops, can pose a high risk of N losses after harvest due to
substantial quantities of readily mineralizable N in crop residues. Organic C amendments
(OCAs) may reduce N losses via immobilization, however, the synchrony of OCA
decomposition and cole crop residue N mineralization is crucial. A soil incubation study
evaluated net N and C mineralization of broccoli residue-derived N or fertilizer-derived N with
three OCAs: wheat straw, yard waste, or used cooking oil, to predict N immobilization and the
potential to mitigate post-harvest N losses. By the 56 d of incubation, broccoli residue
mineralized 67.0 mg N kg-1. In the broccoli residue-derived N treatments, wheat straw, yard
waste, and used cooking oil significantly reduced the quantity of net N mineralization by 16.9,
12.3, and 86.0 mg N kg-1, respectively. The net N mineralization data were fitted to a first-order
exponential model, and the overall trend of OCA was negative, indicating immobilization,
whether N was derived from broccoli residue or fertilizer. The order of effect from OCAs on N
immobilization corresponded to the order of effect on net C mineralization, where wheat straw
and yard waste were lower than used cooking oil. In broccoli residue treatments, compared to
fertilizer, higher N immobilization occurred for used cooking oil, and higher net C
mineralization occurred for used cooking oil and yard waste. The higher N immobilization and
net C mineralization suggests that broccoli residue produced a synergistic effect on the
decomposition of used cooking oil. Additionally, both broccoli residue and used cooking oil
treatments had synchronous peaks of net C mineralization at 4 d. This study provides evidence to
28
warrant field studies to confirm that the application of organic C, especially used cooking oil,
after cole crop harvest may be a beneficial management practice to minimize soil N losses.
2.2 INTRODUCTION
Nitrogen amendments are frequently used to enhance crop production yet N is naturally
susceptible to losses via nitrate leaching, denitrification, and ammonia volatilization. These
losses lead to groundwater and atmosphere contamination which have a negative impact on the
environment. The greatest risk for N losses when producing many vegetable crops may be after
harvest, due to the large quantities of N in the crop residue (Everaarts and De Willigen 1999a;
Neetson et al. 1999; Bakker et al. 2009a). While much research has been done to reduce N losses
during the crop growing season (Everaarts 1993; Thompson et al. 2002; Bakker et al. 2009a),
there is need to minimize N losses during the post-harvest season, when the risk of N losses is
much greater.
Cole crops in particular, can pose a high risk of N losses during the post-harvest period,
due to the large quantities of readily mineralizable N in the crop residue. Optimal broccoli yields
are obtained with high N fertilizer rates, such as 270 to 550 kg N ha-1 (Zebarth et al. 1995;
Everaarts and De Willigen 1999b; Thompson et al. 2002; Yoldas et al. 2008; Bakker et al.
2009b). Thus applications of 400 to 500 kg N ha-1 are not atypical by growers. Broccoli plants
accumulate large quantities of N, up to as much as 320 and 401 kg N ha-1 in aboveground
biomass with optimal N fertilizer rates (Thompson et al. 2002; Bakker et al. 2009a),
consequently leaving ≈ 100 to 330 kg N ha-1 in the field as crop residue or SMN (Everaarts and
De Willigen 1999a; Bakker et al. 2009a). Potential post-harvest mineral N losses appear to be
more related to crop residue N rather than N fertilizer remaining in the soil, as SMN at broccoli
29
harvest typically ranges from 4 to 7 mg N kg-1 in the top 30 cm depth with N fertilizer rates from
0 to 450 kg N ha-1 (Bakker et al. 2009a). Additionally, Everaarts and De Willigen (1999a)
reported mineral N concentrations of 12 to 34 kg N ha-1 in 0-60 cm depth at harvest after the
optimum banded N application (270 kg N ha-1). Broccoli residue decomposition results in rapid
N mineralization which fits a first-order kinetics model and releases ≈ 35 to 60 % of total N in
broccoli residue (De Neve and Hofman 1996). Thus, based on the 35 to 60 % N release of
broccoli residue (containing 100 to 330 kg N ha-1), it is estimated that 35 to 198 kg N ha-1 would
be mineralized after harvest. Considering that the quantity of SMN is directly related to the
vulnerability for N losses (Addiscott et al. 1991), broccoli residue poses a significant risk of N
losses during post-harvest period due to the large quantity of mineralizable N.
A management practice of amending soil with organic C has the potential to immobilize
N, thereby minimize N losses in agricultural systems. By redirecting organic C materials from
waste streams, a new sustainable method of utilizing these materials may be developed. Research
has demonstrated OCAs of wheat straw (Jawson and Elliot 1986; Mary et al. 1996; Bending and
Turner 1999; Henriksen and Breland 1999), yard waste (Hartz and Giannini 1998; Amlinger et
al. 2003; Claassen and Carey 2004), and oily food waste (Plante and Voroney 1998; Rashid and
Voroney 2003; Rashid and Voroney 2004) reduced SMN levels via N immobilization. In the
aforementioned studies, immobilized N was derived from fertilizers or indigenous SMN. There
is potential to immobilize N derived from crop residue with the addition of organic C, however
the timing and rate of crop residue N mineralization relative to organic C decomposition will
affect the mineral N dynamics. Co-application of a high N crop residue (groundnut) with a high
C crop residue (rice straw) has been shown to reduce N losses (Kaewpradit et al. 2009).
30
The application of organic C with broccoli residue has not been investigated. The
decomposition rates for broccoli residue alone, broccoli residue-derived N with organic C, and
fertilizer-derived N with organic C must be investigated to assess the effectiveness of reducing
SMN levels after broccoli harvest. Therefore, the objectives of this study were to evaluate the
effects of three different OCAs (wheat straw, yard waste, and used cooking oil) on N dynamics
(N mineralization) and microbial activity (C mineralization) under controlled laboratory
conditions, when the source of N was derived from broccoli residue or fertilizer. This research
could lead to the development of best management practices for more sustainable cole crop
production by minimizing post-harvest soil N losses.
2.3 MATERIALS AND METHODS
Brookston clay loam (Orthic Humic Gleysol) soil from Ridgetown, Ontario previously
cropped to broccoli was collected from 0-15 cm depth at harvest in 2010. Soil characteristics
were evaluated according to Carter and Gregorich (2008), which included pH (1:1 v v-1 method),
organic matter (loss on ignition), N (KCl extraction and colorimetric analysis method), P (Olsen
bicarbonate extraction method), Ca, K, Mg (atomic absorption via ammonium acetate
extraction), cation exchange capacity (CEC) (estimated based on ammonium acetate extraction
and pH), and soil texture (hydrometer method). The soil used in microcosms had a pH of 5.2, 28
000 mg of organic matter kg-1, 9.5 mg N kg-1, 35 mg P kg-1, 1984 mg Ca kg-1, 66 mg K kg-1,128
mg Mg kg-1, CEC of 20.8 cmol kg-1, 58% sand, 18% silt, and 24% clay. Visible roots were
removed from the soil sample by hand-picking and passing the soil through a 5mm screen. A
soil subsample was dried at 60°C for moisture content determination and preparation for the
incubation study.
31
After harvest, eight broccoli plants (head removed, stem and leaves intact) were collected
from the field. Intact plants were oven-dried at 60°C for moisture content determination and
preparation for the incubation study, and dried tissue was hand chopped into < 1 cm2. Broccoli
stem pieces that were too tough to chop by hand were ground using a Wiley Mill (Thomas
Scientific, Swedesboro, NJ, USA) with a 2 mm diameter opening screen mesh. Broccoli stem
and leaf pieces were homogenized prior to the addition to soil.
The OCAs included locally sourced (Ridgetown, Ontario): recently harvested wheat
straw, municipal yard waste, and restaurant used vegetable cooking oil. Solid OCAs were oven
dried at 60°C for moisture content determination, and hand chopped to approximately < 1 cm2
for use in the incubation study. The used cooking oil was mixed thoroughly prior to application
to soil. The C:N ratios and N content of broccoli residue and OCAs (Table 2.1) were determined
by dry combustion (Rutherford et al. 2008), using a LECO CN analyzer (Leco Corporation, St.
Joseph, MI, USA).
Table 2.1 The C:N ratio and N content (mg kg-1 dry matter) of the amendments z.
N Content
Amendment
C:N Ratio
(mg kg-1 dry matter)
Broccoli residue
10:1
39000
Wheat straw
61:1
7000
Yard waste
52:1
6000
Used cooking oil
>1000:1
400
z
Dry matter for solid materials and as-is for used cooking oil.
32
2.3.1
Nitrogen Mineralization
For the N mineralization trial, a randomized complete block design was established with
the experimental units randomized within four replications in each of eight time blocks, for
destructive sampling of microcosms. The microcosms were set up in 250 mL glass jars each
containing 60.0 g of dry soil. The soil was adjusted to 40% water holding capacity, as estimated
by soil texture (Saxton et al. 1986), by adding 7.20 mL distilled water to each jar, and preincubated for 3 d before any amendments were added. The broccoli residue-derived N treatments
included: an unamended control, a control of 268 mg of dry broccoli residue (without any other
amendments), and 268 mg of dry broccoli residue + each OCA (separate OCA treatments with
weights equivalent to 207 mg of fresh weight, (thus a dry weight of 177 mg of wheat straw, 157
mg of yard waste, and a fresh weight of 207 mg for used cooking oil). Additionally, fertilizerderived N treatments were established to serve for comparison to the broccoli residue-derived N
treatments. In incubation studies, mineral N is typically added to establish sufficient SMN levels
for microbial decomposition, such as in Rashid and Voroney (2004). Thus, to investigate the
timing of broccoli residue N mineralization in relation to the microbial N requirement for organic
C decomposition, the fertilizer-derived N treatments included: fertilizer only (with 34 mg of
NH4NO3), 268 mg of dry broccoli residue + 34 mg of NH4NO3, and 34 mg of NH4NO3 + each
OCA, at the respective, aforementioned rate.
The rate of broccoli crop residue application corresponds to 60 Mg ha-1 fresh crop
residue, a rate representative of field broccoli residue (Bakker et al. 2009a) yet greater than used
in previous laboratory research (De Neve et al. 1996). The rate of OCA corresponds to a field
application rate of 5 Mg ha-1 fresh organic carbon material and within the recommendations set
33
by Rashid and Voroney (2004) for used cooking oil. The amounts of wheat straw, yard waste,
and used cooking oil can be expressed as quantities of added C, with C loading rates of 1.2, 0.9,
and 1.5 g C kg-1 soil, respectively, and reflect a practical application rate at a field scale. The
quantity of NH4NO3 fertilizer added to attain sufficient levels of SMN was the calculated
microbial N requirement (50 mg N kg-1) for total decomposition of the broccoli residue, based on
a microbial C:N ratio of 8:1, a C assimilation of one third added C, an average C loading rate of
1.2 g C kg-1 soil, and assuming negligible N will be supplied by OCAs.
At the incubation time zero (which was 3 d after soil wetting and pre-incubation), soil
and amendments were mixed thoroughly to ensure homogeneity before the jars were closed with
perforated gas-permeable Parafilm to minimize water loss. The microcosms were incubated in
the dark, at room temperature (22°C±3°C). The jars were weighed weekly to assess moisture
loss; distilled water was added approximately every two weeks to maintain soil moisture.
The microcosms were destructively sampled by removing four replicates of each
treatment in one time block on 1, 3, 7, 14, 21, 28, 42, and 56 d. Soil NO3--N and NH4+-N were
quantified using the Maynard et al. (2008) method, briefly, 5 g of soil was extracted with 25 mL
of 2 M KCl, shaken for 30 minutes, filtered and analyzed colorimetrically on an AutoAnalyzer3
(SEAL Analytical Inc., Mequon, WI, USA) with a high resolution digital colorimeter to quantify
NH4+-N (method G-102) and NO3--N (method G-200). Nitrogen mineralization was expressed
as a value in mg N kg-1 soil. Net N mineralization of: (i) broccoli residue alone treatment was
estimated as the difference of SMN in the broccoli residue alone and SMN in the unamended
treatment, (ii) broccoli residue-derived N with OCA was estimated as the difference of SMN in
broccoli residue-derived N with OCA and SMN in the unamended treatment, (iii) fertilizer-
34
derived N with OCA (or broccoli residue) treatments was estimated as the difference of SMN of
fertilizer-derived N with OCA (or broccoli residue) treatments and SMN in fertilizer alone
treatment.
2.3.2
Carbon Mineralization
The design of the C mineralization trial was a randomized complete block with four
replications. The treatments, soil quantity, and amendment weights were the same as the N
mineralization trial. To ensure good aeration, soil was placed in a 250 mL volume container
made of 1 mm2 mesh screen aluminum, and sealed inside a 2 L glass jar. In each jar was a vial
containing 20 mL of 1 M NaOH to trap evolved CO2 (Hopkins et al. 2008) and a 20 mL vial of
distilled water to maintain soil moisture. A set of two sealed 2 L jars with only NaOH and water
vials were included to quantify background CO2 levels. All microcosms were incubated in the
dark at 22°C±3°C.
Non-destructive sampling occurred on 2, 4, 7, 10, 13, 17, 22, 28, 35, 42 and 56 d.
Evolved CO2 was determined by back-titrating NaOH with 0.5 M HCl using a phenolphthalein
indicator, after the addition of 2 mL 1 M BaCl2. At each sampling day, the jars were opened for
30 to 60 min before replacing the NaOH traps and resealing the jar. Net C mineralization was
expressed as a cumulative value in mg C kg-1soil, as well as a rate of mg C kg-1 soil d-1. Net C
mineralization was calculated in the same method as described for net N mineralization.
2.3.3
Statistical Analyses
The analyses were conducted with the PROC MIXED method in SAS (SAS Institute,
version 9.2, Cary, NC, USA). The net N mineralization data were subjected to an analysis of
35
variance with fixed effects of treatment, incubation day, and treatment x incubation day
interaction. Random effect was block (incubation day). The assumptions of a variance analysis
were that model effects were additive and the errors were random, independent, normally
distributed, mean of zero, and homogeneous. Assumptions were tested by plotting residuals of
predicted by fixed effects and assessing normality with a Shapiro-Wilk test (Bowley 2008). The
presence of outliers was tested by comparing studentized residuals to Lund’s critical value
(Bowley 2008). Treatment x incubation day interaction was partitioned into simple effects of
treatment at the incubation day level of 56 d (final day) with an LSD test, and regression
contrasts with an F-test. Significance was determined with a type 1 error rate of 0.05. A nonlinear regression for a first order exponential model y=A(1-exp-k(t)), (y= net SMN, A=coefficient
indicates amount of mineralizable N at 1 d, k=rate constant, t= incubation day) was fitted to the
net N mineralization data and equation coefficients were determined with PROC NLIN. The net
C mineralization and C mineralization rate data were subjected to a repeated measures analysis
of variance with fixed effects of treatment, incubation day, and treatment x incubation day
interaction. Random effect was block. The same assumptions of an analysis of variance were
tested, as described above. Repeated measures analysis was conducted with treatment x block as
the subject. The model included the generalized Kenward-Roger correction to compute
denominator degrees of freedom (Littell et al. 2002). The default covariance structure of the
response variable was chosen to render convergence of the complex model. To assess the
predictive strength of a logistic regression, Efron's Pseudo R2 (Efron 1978) was computed as the
square of the correlation between the predicted and observed values. Net C mineralization rate
data were partitioned into simple effects of treatment 4 d (peak) with an LSD test, and net C
36
mineralization data were partitioned into simple effects of treatment 56 d (final day) with an
LSD test. Significance was determined with a type 1 error rate of 0.05.
2.4 RESULTS AND DISCUSSION
2.4.1
Mineral Nitrogen Concentrations
In the unamended control, SMN was 37.5 mg N kg-1 (4 d after rewetting) and continued
to increase throughout the study. Numerous studies have found a SMN flush after re-wetting soil
which was oven-dried (Birch 1959; Blagodatsky and Yevdokimov 1998). A mineral N spike can
be attributed to the drying of soil which disturbs microbial biomass, causes cellular lysis, and
consequently a release of N from microbial biomass (Hans-Werner et al. 2004), or N
mineralization of broccoli roots. Thus, the flush of SMN on day 1 of the incubation in the
unamended control observed in the present study was likely a reflection of soil drying prior to
the experiment. Hence, all data were presented as net mineralization.
Throughout the time course, NH4+-N levels were higher than expected (an average of
56% of SMN) considering that NH4+ is usually rapidly converted to NO3- in most agricultural
soils (Norton 2008). The apparent low nitrifying activity was likely attributed to soil drying at
60oC prior to the experiment, as well as the soil pH of 5.2. High soil temperatures (>35°C) can
slow nitrifying activity (Myers 1975; Hasson et al. 1987), and intermediate temperatures (15 to
25°C) have resulted in maximum nitrification (Avrahami et al. 2003). However, some
differential responses of soil organisms after heating had led to the suggestion that
chemoautotrophic nitrifiers may be less sensitive to heat than currently believed (Hart et al.
2005). Bartlett and James (1980) acknowledge that oven-drying soil can be most convenient for
laboratory practices, and soil may be re-moistened for a period prior to use. Fu et al. (1987) also
37
found significant NH4+ levels during 12 weeks of incubation in one soil at pH 4. The relatively
low soil pH (5.2) in the present study may have inhibited nitrification, as it generally slows
below pH 6 (Paul and Clark 1996). Similarly, a sharp rise in NH4+ concentration has been
observed in dried and re-wetted acidic soils (pH 4.45), with a slow recovery of nitrifying activity
(Mian et al. 2008). Fungi produce acid proteinases involved in formation of NH4+ and are active
at pH 5 (Myrold and Bottomley 2008). Visual observations of mycelia growth support the
presence of fungi in the current study. Given that significant quantities of both NH4+-N and NO3-N were present, the mineralization of N was based on the SMN content rather than NO3--N
alone. The effects of treatment, day, and treatment x day interaction was significant for net N
mineralization (P<0.0001). The interaction signifies that the treatments produced different effects
on N mineralization over time, likely a reflection of N immobilization vs. mineralization rates in
different treatments. Because there was a significant interaction, a model was fitted to data from
each treatment and the differences among treatments were also assessed on the 56 d of
incubation.
2.4.2
Net Nitrogen Mineralization
By 56 d, broccoli residue alone resulted in the net mineralization of 67.0 mg N kg-1, and
the addition of fertilizer to broccoli residue did not significantly alter net N mineralization (Table
2.2). Therefore, the quantity of N released from broccoli residue appears to be independent of the
initial SMN levels. Based on total N in broccoli residue, 37% was apparently mineralized by 56
d. Similarly, De Neve and Hofman (1996) studied apparent N mineralization from broccoli
residue parts (leaf blades, upper stem and lower stem) which ranged from ≈ 35-60 % of total N,
after 105 d of incubation. The sharpest rise in apparent broccoli N mineralization was in the first
38
1 to 3 d (De Neve and Hofman 1996). Accordingly, by 1 d, 18% of total broccoli residue N was
apparently released as mineral N in the current study.
Table 2.2 The effect of organic carbon amendment with broccoli residue-derived N or fertilizerderived N on net N and C mineralization at 4 or 56 d of incubation z.
Net C mineralization rate
Net N mineralization
Net C mineralization
at 4 d
at 56 d
at 56 d
(mg C kg-1 dry soil d-1)
(mg N kg-1dry soil)
(mg C kg-1 dry soil)
Broccoli
Amendment
NH4NO3
Broccoli
NH4NO3
Broccoli
NH4NO3
residue
residue
residue
Broccoli residue
10.6 b
10.0 b
73.5 a
67.0 a
68.5 c
66.2 c
Wheat straw
<0.1 d
<0.1 d
-28.3 c
-16.9 bc
25.1 d
33.6 d
Yard waste
<0.1 d
1.44 c
-9.9 b
-12.3 b
4.00 e
20.7 d
Used cooking oil
12.2 a
11.8 a
-66.2 d
-86.0 e
155 b
176 a
Standard error
z
0.35
4.57
5.52
For each mineralization section, means followed by the same letter were not significantly different
according to an LSD test of simple treatment effects.
After 56 d, net N mineralization of broccoli residue alone (67.0 mg N kg-1) was
significantly lowered by the amendments of wheat straw, yard waste, and used cooking oil to 16.9, -12.3, and -86.0 mg N kg-1, respectively (Figure 2.1, Table 2.2). The effect of OCA on net
immobilization of broccoli residue-derived N followed the order of: used cooking oil > wheat
straw = yard waste, which corresponds to the trend from highest to lowest C:N ratio (Table 2.1)
39
and the quantities of C added in each OCA. The order may therefore be a function of the
decomposability of the OCA.
When the source of N was derived from broccoli residue compared to fertilizer, net N
immobilization was similar for the wheat straw and yard waste treatments yet immobilization
was higher in the used cooking oil treatment (Table 2.2). Compared to fertilizer, it appears that
broccoli residue facilitated a synergistic effect on net N immobilization with used cooking oil,
probably due to stimulation of microbial activity by the broccoli residue and/or microbial luxury
N consumption. Mary et al. (1996) suggested that differences in microbial N assimilation exist,
and were likely due to the type of decomposing microflora. Thus, it was possible that the
suggested synergistic effect on N immobilization occurred for used cooking oil but not for wheat
straw or yard waste due to the different microflora involved in primary decomposition of each
OCA.
Although the result of net N immobilization with OCA application was consistent with
other incubation studies, the level of N immobilization appears lower than previous research. Net
N immobilization was 53-65, 20-50, and 200 mg N kg-1 after the addition of maize straw
(Recous et al. 1995), compost (Hartz and Giannini 1998), and used cooking oil (Plante and
Voroney 1998), respectively.
40
120
80
80
Net N Mineralization
(mg N kg-1 soil)
Net N Mineralization
(mg N kg-1 soil)
120
40
0
-40
-80
40
0
-40
-80
-120
-120
1
8
15
22 29 36
Time (d)
43
50
1
Broccoli residue
Broccoli residue + wheat straw
Broccoli residue + yard waste
Broccoi residue + used cooking oil
8
15
22 29 36
Time (d)
43
50
Broccoli residue + ammonium nitrate
Ammonium nitrate + wheat straw
Ammonium nitrate + yard waste
Ammonium nitrate + used cooking oil
Figure 2.1 Cumulative net N mineralization (mg N kg-1dry soil) as affected by organic C amendment with broccoli residue-derived N (left)
or fertilizer-derived N (right), during the 56-d incubation. Data points represent the mean of observed values (n=4) and bars represent the
standard error of the mean but may be smaller than the symbols.
41
Net N mineralization data were fitted to a first-order exponential model (Table 2.3).
Unamended and N fertilizer alone treatments released 25.7 and 21.4 mg N kg-1 throughout
incubation, according to the equations SMN= 56.8(1-exp-1.762(t)) and 74.8(1-exp-1.389(t)),
respectively. The present estimated k coefficients for net N mineralization were 0.31 and 0.14
from broccoli residue alone or broccoli residue with fertilizer N, respectively (Table 2.3).
Previous research found broccoli residue mineralization fitted a similar first-order kinetic model,
where rate constant values for separate broccoli plant parts (leaf blades, upper stem, lower stem)
ranged from 0.635 to 1.697 (De Neve and Hofman 1996). The discrepancy between De Neve and
Hofman (1996) and our k constants may be due to fresh residue incorporation in non-air dried
soil compared to dry residue incorporation in dried and re-wetted soil, respectively. Drying soil
retards the decomposition of added materials (Magid et al. 1999), thus supporting the slower
decomposition indicated by the lower rate constants in the present study compared to De Neve
and Hofman (1996).
42
Table 2.3 Effect of organic carbon amendment with broccoli residue-derived N or fertilizer-derived
N on net N mineralization in the 56 d incubation.
Model Parametersz
----------A----------
----------k---------Pseudo
N source
C amendment
y
Value
95%CL
Value
y
95%CL
R2
Broccoli residue
68.3
62.7 to 73.9
-34.9 to -18.9
0.31
0.18 to 0.43
0.62
0.11
-0.010 to 0.23
0.45
0.031
-0.050 to 0.11
0.29
Broccoli residue
Wheat straw
-26.9
Broccoli residue
Yard waste
-16.4
Broccoli residue
Used cooking oil
-102 -109 to -94.4
0.27
0.18 to 0.37
0.81
NH4NO3
Broccoli residue
70.6
0.14
0.090 - 0.20
0.75
NH4NO3
Wheat straw
-29.7 -36.0 to -23.5
0.10
0.030 to 0.17
0.56
NH4NO3
Yard waste
-11.9 -16.5 to -7.30
0.061
-39.8 to 7.04
63.8 to 77.5
-0.00040 to
0.50
0.12
NH4NO3
z
Used cooking oil
-73.7 -78.0 to -69.3
0.30
0.21 - 0.38
Fitted to a first order exponential model y = A(1-exp-k(t)), (y= SMN, A=coefficient, k=rate constant, t=
day). All models presented were significant at P<0.0001.
y
Upper and lower confidence limits.
Positive net N mineralization occurred in the broccoli residue alone and broccoli residue
with fertilizer N, while negative net N mineralization, indicating N immobilization, occurred in
broccoli residue-derived N and fertilizer-derived N with each OCA (Figure 2.1; Table 2.3).
Therefore, the overall trend in net N mineralization with OCA was negative whether N was
derived from broccoli residue or fertilizer. Similar to the patterns for net N mineralization in this
43
0.89
study, previous research demonstrated net N immobilization with application of wheat straw
(Jawson and Elliot 1986; Mary et al. 1996; Bending and Turner 1999; Henriksen and Breland
1999), net immobilization of 39 mg N g-1 C added as maize straw (Recous et al. 1995), 1.2 to 5
mg N net immobilized per month after the application of compost yard waste (Claassen and
Carey 2004), and rapid N immobilization within 2 to 4 d of application of used cooking oil
(Plante and Voroney 1998).
In the broccoli residue-derived N with used cooking oil treatment, net N mineralization
increased after 21 d (Figure 2.1) suggesting re-mineralization of N from microbial biomass.
Although the exponential model significantly fitted the data from 1 d to 56 d for the broccoli
residue-derived N with used cooking oil treatment (Table 2.3), a linear model of: y=-127 +
0.76(t), (y= net SMN, t= incubation day), significantly fitted the data from 21 d to 56 d (r2= 0.31,
P=0.0242). Thus, N re-mineralization occurred after 21 d. It was possible that a reduced quantity
of easily oxidizable C or available N remained after 21 d which triggered microbial death and
turnover in the broccoli residue-derived N with used cooking oil treatment. The pattern of N
immobilization followed by re-mineralization with cooking oil was consistent with other
laboratory (Smith 1974) and field (Rashid and Voroney 2003) studies. If the period of N
immobilization coincides with periods of high risk for N losses in the field, a reduction in N
losses may result. Future research should use field studies to investigate the effects of applying
organic C and assess N losses after cole crop harvest and effects on subsequent crop production.
2.4.1
Net Carbon Mineralization
The effects of treatment, day, and treatment x day interaction were significant (P<0.0001)
for net C mineralization. The interaction signifies that the treatments produced different effects
44
on net C mineralization over time, likely a reflection of the differences in microbial activity with
different treatments. Because there was a significant interaction, analysis focused on the 56 d net
C mineralization measures, as well as the peak rates of net C mineralization. Unamended and
fertilizer alone treatments had negligible C mineralization over the study, (3.45 and 0.82 mg C
kg-1, respectively), suggesting low levels of microbial activity due to the lack of readily
mineralizable C.
The effect of OCAs on net C mineralization followed the order of: used cooking oil >
wheat straw = yard waste with broccoli residue-derived N, and the order of: used cooking oil >
wheat straw > yard waste with fertilizer-derived N (Figure 2.2; Table 2.2). Amendments were
added at an equivalent field rate of 5 Mg ha-1, which resulted in C loading rates of 1.2, 0.9, and
1.5 g C kg-1 soil for wheat straw, yard waste, and used cooking oil, respectively. When net C
mineralization on 56 d was expressed as a proportion of C loading rates, OCAs with broccoli
residue-derived N had net C mineralization of 118, 28, and 23 mg C kg-1, and with fertilizerderived N had 104, 21, and 4.4 mg C kg-1 from used cooking oil, wheat straw, and yard waste,
respectively. Thus, the order of net C mineralization of OCAs was not influenced by the slightly
different C loading rates but reflects the amendment’s decomposability, and corresponds to the
order of effect on N immobilization noted earlier, as well as the descending trend for C:N ratio
(Table 2.1). Based on the C loading rates, the CO2 emitted, and the assumption that microbes
have a C assimilation efficiency of approximately 30%, wheat straw, yard waste, and used
cooking oil were up to 4, 3, and 16 % decomposed after 8 weeks of incubation with broccoli crop
residue.
45
In soil amended with broccoli residue alone, the rate of peak net C mineralization
occurred at 4 d with 10.0 mg C kg-1 d-1 and was not significantly different than broccoli residue
with fertilizer (Table 2.2). When used cooking oil decomposed with broccoli residue-derived N
or fertilizer-derived N, peak net C mineralization also occurred at 4 d, synchronous with that of
broccoli residue alone (Table 2.2). Relative to broccoli residue or used cooking oil, lower rates of
net C mineralization occurred in wheat straw and yard waste treatments with peaks after 7 d.
Therefore, used cooking oil decomposed more rapidly than wheat straw and yard waste. The
interpretation of results was not different if the peak rates were analyzed at 4 d or after 7 d for
wheat straw and yard waste. It should be noted that peak level of net N immobilization in the
used cooking oil treatment coincided with peak rate of net C mineralization, akin to other
research (Plante and Voroney 1998). It therefore appears that used cooking oil, compared to the
other OCAs, stimulated earlier microbial activity which was most synchronous to that of broccoli
residue.
46
Cumulative Net C Mineralization
(mg C kg-1 soil)
Net C Mineralization
(mg C kg-1 soil)
200
160
120
80
40
0
1
8
15
22 29 36
Time (d)
43
200
160
120
50
80
40
0
1
8
15
22 29 36
Time (d)
43
Broccoli residue
Ammonium nitrate + broccoli residue
Broccoli residue + wheat straw
Ammonium nitrate + wheat straw
Broccoli residue + yard waste
Ammonium nitrate + yard waste
Broccoli residue + used cooking oil
Ammonium nitrate + used cooking oil
50
Figure 2.2 Cumulative net C mineralization (mg C kg-1 dry soil) as affected by organic carbon amendment with broccoli residue- derived
N (left) or fertilizer-derived N (right), during the 56-d incubation. Data points represent the mean of observed values (n=4) and bars
represent the standard error of the mean but may be smaller than the symbols.
47
Similarly, Jawson and Elliot (1986) found peak CO2 evolution after 6 d of incubation of
wheat straw added to soil, and Plante and Voroney (1998) found peak C mineralization rate near
4 d for decomposition of canola oil. Peak C mineralization rates from the decomposition of
canola oil were 35 or 52 mg C kg-1 soil h-1 (Plante and Voroney 1998), much higher than that of
the present study. The discrepancy may be related to a lower microbial population establishment
due to the soil drying pre-treatment, or the higher C loading rates of 6 and 13 g C kg-1 soil in
Plante and Voroney’s study (1998) compared to 1.5 g C kg-1 soil in our study. Plante and
Voroney (1998) also found a higher extent of decomposition from oily food waste (36 to 40 %)
after 4 weeks of incubation, compared to the current study.
The yard waste and used cooking oil treatments had a significantly greater quantity of net
C mineralization when supplied with N derived from broccoli residue compared to fertilizer
(Table 2.2). The enhanced net C mineralization with broccoli residue-derived N with used
cooking oil supports the synergistic effect of broccoli residue on net N immobilization, suggested
earlier. Although there was no enhanced N immobilization for yard waste with broccoli residuederived N compared to yard waste with fertilizer-derived N, the net C mineralization data
suggests that broccoli residue enhanced microbial activity during decomposition of yard waste,
relative to fertilizer (Table 2.2). Considering that yard waste is composed of more structural
components, such as lignin, it likely had a greater quantity of less easily oxidizable C than the
other OCAs. Thus, the addition of readily decomposable broccoli residue may have stimulated
microbial growth and consequently promoting C mineralization of an otherwise more recalcitrant
material, yard waste.
48
2.4.2
Overall Nitrogen and Carbon Dynamics
The quantity and rate of net N and C mineralization is dependent on the type of substrate,
its C:N ratio, and quantity of C applied. In the present study, used cooking oil had the highest
C:N ratio (Table 2.1), and the most rapid decomposition. It is expected that the nature and
particle size of each OCA influences N dynamics. Bending and Turner (1999) have found that
increasing surface area available for colonization induces earlier utilization of substrate by soil
microbes. Liquid used cooking oil would have greater exposed surface area in soil compared to
either solid wheat straw or yard waste, which may have contributed to earlier microbial activity
in treatments containing used cooking oil compared to the other OCAs (Table 2.2). Another
possible explanation for the differences in N and C dynamics resulting from different OCA
treatments may be that different types of microflora were involved in the primary decomposition
of each substrate (Mary et al. 1996).
2.5 CONCLUSION
Broccoli residue appears to have had a synergistic effect on N immobilization with used
cooking oil, and enhanced net C mineralization of both yard waste and used cooking oil. The
synergistic effect suggests that a greater capacity of N immobilization and microbial activity may
result when the organic C is incorporated with broccoli residue. Nitrogen re-mineralization of
used cooking oil with broccoli residue should be explored with field studies to predict an optimal
time of used cooking oil application, the potential to minimize post-harvest N losses, and assess
latent effects on subsequent crops. Based on the presented results, field experiments are
recommended to examine wheat straw, yard waste, or used cooking oil as a management practice
for reducing post-harvest N losses in cole crop production. Used cooking oil presents the most
49
promise due to greater levels of net N immobilization and the synchronous peak net C
mineralization with broccoli residue.
50
3
EVALUATION OF POST-HARVEST ORGANIC CARBON AMENDMENTS AS A
STRATEGY TO MINIMIZE NITROGEN LOSSES IN COLE CROP PRODUCTION.
3.1 ABSTRACT
Cole crops can pose a significant risk for N losses during the post-harvest period due to
substantial amounts of readily mineralizable N in crop residues. Amending the soil with organic
C has the potential to immobilize N and thereby reduce the risk for N losses. Four field trials
were conducted to determine the effects of OCA on N dynamics and spring wheat harvest
parameters proceeding early- and late-broccoli systems in 2009 and 2010. The experimental
controls represented the traditional grower practice of incorporated broccoli crop residue (CRcontrol) and the pre-plant application of N fertilizer (CRN-control) to subsequent spring wheat.
Alternative practices were compared to the controls, which included broccoli crop residue
removal (CR-removal), an oat (Avena sativa L.) cover crop (CC-oat), and three different OCA of
wheat straw (OCA-straw), yard waste (OCA-yard), or used cooking oil (OCA-oil). The
treatments, which demonstrated reduced autumn SMN concentrations after broccoli harvest,
relative to the CR-control, were CR-removal, OCA-straw, and OCA-oil. Although CR-removal
and OCA-straw indicated a reduced potential for autumn soil N losses in the early-broccoli
system, these practices are not recommended for growers because subsequent spring wheat yield
and profit margins were reduced compared to the CR- and CRN-controls. The OCA-oil reduced
autumn SMN concentrations by 53 to 112 kg N ha-1 relative to the CR-control after both earlyand late-broccoli harvest, suggesting a larger potential for reduced autumn soil N losses,
compared to all other treatments. No detrimental effects resulted from the OCA-oil treatment on
the subsequent spring yield or grain N. The OCA-oil reduced spring wheat profit margins
relative to the CR-control, like the OCA-straw and CR-removal treatments, however profit
51
margins were similar between the OCA-oil and the CRN-control. Therefore, in areas with a high
risk of environmental N contamination, growers should consider the OCA-oil practice after cole
crop harvest to minimize the risk of N losses.
3.2 INTRODUCTION
Nitrogen fertilizer applications are frequently used to enhance vegetable crop production,
and much research has been done to reduce N losses during the vegetable crop growing season
(Thompson et al. 2002; Bakker et al. 2009a). However, after harvest, a large quantity of N can
remain in vegetable crop residues (De Neve and Hofman 1998; Everaarts and De Willigen
1999a; Thompson et al. 2002), which readily mineralizes (De Neve and Hofman 1998) and may
be susceptible to post-harvest losses. There is need to minimize N losses during the post-harvest
season when the risk of losses is much greater due to the annual water budget in Ontario (Fallow
et al. 2003) and elsewhere. Losses include nitrate (NO3−-N) leaching and denitrification, which
can have negative environmental consequences such as groundwater and atmosphere
contamination. Thus, the development of more sustainable agricultural practices, which are
focused on soil N management after vegetable harvest is necessary.
Cole crops, in particular, produce optimal yields with high N applications, ranging from
270 to 550 kg N ha−1 for broccoli (Zebarth et al. 1995; Everaarts and De Willigen 1999a;
Everaarts and De Willigen 1999b; Yoldas et al. 2008; Bakker et al. 2009b). Cole crop residues
may leave ≈100 to 330 kg N ha−1 in the field at harvest (De Neve and Hofman 1998; Thompson
et al. 2002), and post-harvest mineral N losses are more related to crop residue N rather than N
fertilizer remaining in the soil (De Neve and Hofman 1998). Considering that 35 to 60% of
broccoli crop residue N has been found to mineralize in controlled incubation studies (Chaves et
52
al. 2007a; Congreves et al. 2013b), and that the crop residue may contain up to 330 kg N ha−1
(Bakker et al. 2009a), then up to 198 kg N ha−1 would be mineralized in the field after harvest
from broccoli crop residue (Congreves et al. 2013b). Thus, cole crop residue poses a significant
risk for N losses due to the large quantity of mineralizable N in the post-harvest season.
Amending the soil with organic C material has the potential to reduce soil mineral N
(SMN) concentrations through N immobilization (Chaves et al. 2007a; Congreves et al. 2013b).
By redirecting organic C materials from waste streams, a new sustainable method of utilizing
these materials could be developed. Some high C materials, which are expected to be readily
available for vegetable producers in Ontario, are wheat straw, yard waste, or used cooking oil.
For example, winter wheat typically has 1.4 Mg ha−1 of straw residue (Oo 2012). Up to 326,000
ha−1 of winter wheat was harvested in Ontario in 2010 (OMAFRA 2010). Also, 400,000 Mg yr−1
of leaf and yard waste has been estimated from Ontario residential collection (Van der Werf
2009), and approximately 450,000 Mg of oily food waste is produced annually in Ontario
(Rashid and Voroney 2004; ORMI 2013). Considering that the estimated total production of
broccoli, cabbage, and cauliflower in Ontario was on 4147 and 4247 ha−1 in 2010 and 2011
(OMAFRA 2012a), respectively, there is potential for incorporating the organic C waste
materials as amendments for N management after cole crop production.
Research has demonstrated that SMN concentrations may be reduced via N
immobilization by the applications of wheat straw (Jawson and Elliot 1986; Mary et al. 1996;
Bending and Turner 1999), yard waste (Hartz and Giannini 1998; Amlinger et al. 2003; Claassen
and Carey 2004), and oily food waste (Plante and Voroney 1998; Rashid and Voroney 2003;
Rashid and Voroney 2004). In the aforementioned studies, immobilized N was derived from
53
fertilizers or indigenous SMN. The synchrony of cole crop and organic C material decomposition
is crucial for the immobilization of N derived from crop residue. In an incubation study, N
derived from broccoli crop residue was immobilized by the addition of wheat straw, yard waste,
and used cooking oil (Congreves et al. 2013b). The rapid decomposition and synchrony of the
crop residue N mineralization rate relative to the organic C amendment (OCA) decomposition
suggested promise for field application (Congreves et al. 2013b).
Even though a reduced potential for N losses may result if the period of N immobilization
coincides with periods of high risk for N losses in the field, it is necessary to assess potential
effects of OCA on the subsequent crop. Early- rather than late-season N fertilizer input is
recommended to achieve a desired yield goal for spring wheat production (Cassman et al. 1992).
Therefore, if N immobilization due to the OCA after cole crop harvest is not followed by remineralization early enough in the spring wheat growing season, a negative effect on spring
wheat yield could result. Conversely, if N mineralization is synchronous with spring wheat N
demand, then N use efficiency may be enhanced.
A pattern of N immobilization followed by re-mineralization has been found in previous
laboratory (Congreves et al. 2013b) and field (Rashid and Voroney 2003) research after the OCA
of used cooking oil. However, other researchers did not observe re-mineralization in the
subsequent spring after autumn incorporation of cereal straw or green compost with cauliflower
crop residue (Chaves et al. 2007a). Research has found corn, lettuce, or leek production to be
unaffected by the previous autumns’ application of oily food waste (Rashid and Voroney 2004),
cereal straw or waste compost (Chaves et al. 2007a), respectively. Thus, it may be possible to
54
reduce potential N losses during periods of high risk for N losses by applying an OCA, without
negatively affecting the subsequent crop.
In addition to the environmental impacts of N management practices, consideration must
be given to the economic impacts. Some studies have found a trade-off to exist between
environmental and economic benefits because practices that reduced N losses did not have
favorable economic outcomes in vegetable production (Thompson et al. 2000; Saleh et al. 2007).
Other studies have found an overlap for the optimal environmental and economic outcomes (Pier
and Doerge 1995; Thompson and Doerge 1996). Thus, ambiguity exists in the literature with
respect to the economic impacts associated with management practices for reducing N losses.
Regardless, the economic implications of proposing a better management practice to minimize N
losses should be evaluated.
Therefore, the objective of this field study was to evaluate the effects of three different
OCAs of wheat straw (OCA-straw), yard waste (OCA-yard), and used cooking oil (OCA-oil) on
N dynamics, spring wheat production, and profit margins following broccoli harvest, compared
to typical grower practices. This research could lead to the development of better N management
practices, leading to more sustainable cole crop production by minimizing environmental N
contamination.
3.3 MATERIALS AND METHODS
The field sites at Ridgetown Campus in 2009–2010 and 2010–2011 were on a Brookston
clay loam (Orthic Humic Gleysol) soil (Table 3.1). The soil characteristics evaluated (Table 3.1)
included pH (1:1 v/v method), organic matter (loss on ignition), N (KCl extraction and
55
colorimetric analysis method), P (Olsen bicarbonate extraction method), Ca, K, Mg (atomic
absorption via ammonium acetate extraction), cation exchange capacity (CEC) (estimated based
on ammonium acetate extraction and pH), and soil texture (hydrometer method) (Carter and
Gregorich 2008).
Table 3.1 Initial soil characteristics of the experimental sites prior to broccoli transplanting in 2009
and 2010.
Characteristic
2009
2010
pH
6.1
5.6
Soil texture
Loam
Sandy Clay Loam
Sand:Silt:Clay (%)
46:28:26
58:18:24
OM (Mg ha−1)
55
80
CEC (cmol kg−1)
15
23
Bulk density (g·cm−3)
1.4
1.4
N
5
41
P
34
37
K
108
87
Mg
153
149
Ca
2433
2430
(soil sample depth 15 cm)
Pre-plant nutrients (mg·kg−1)
56
Broccoli (c.v. ‘Ironman’) was grown in two systems: Early- and late-harvest, which was
followed by spring wheat production in the subsequent year. The experimental design was a
randomized complete block with four replications within the early- and late-broccoli systems.
Nitrogen was uniformly hand-applied at the typical grower rate of 342 kg N ha−1 as urea, and
incorporated by disking before broccoli was mechanically transplanted on May 17 and 23 for the
early system and June 23 and 23 for the late system in 2009 and 2010, respectively. Early- and
late-broccoli systems were grown in the same field, but considered as separate research trials.
Typical management practices were followed for pre-plant fertilization of macronutrients, plant
spacing (75 cm between rows and 30 cm between plants), insecticide application (LambdaCyhalothrin (13.1%) 19 mL ha−1), and drip irrigation, as required. Temperature and precipitation
data were obtained by the on-site weather station (Table 3.2).
57
Table 3.2 Monthly total precipitation and mean temperature and the 30-yr mean at Ridgetown, ON
during 2009–2011.
Total precipitation (mm)
2009
2010
2011
Mean temperature (°C)
30 yr
2009
2010
2011
mean
30 yr
mean
Jan
147
227
61
61
−10.1
−5.5
−7.5
−3.7
Feb
106
98
137
54
−3.8
−4.8
−6.1
−2.4
Mar
106
67
88
60
1.1
3.4
−0.3
2
Apr
152
63
134
78
7.8
9.8
6.6
8.3
May
49
114
153
75
13.0
14.4
14.0
14.8
Jun
65
97
75
83
17.3
19.3
18.8
20.2
July
31
121
70
86
18.5
22.6
23.5
22.5
Aug
92
19
70
86
19.6
21.2
20.5
21.4
Sept
36
80
135
93
16.1
16.2
16.5
17.6
Oct
70
78
79
69
8.6
10.8
10.4
11.2
Nov
30
92
140
75
6.2
4.2
6.9
4.8
December
138
45
86
67
−2.5
−4.8
1.6
−1.2
Total
1022
1101
1228
887
The early-broccoli system was harvested on August 4 and August 3, while the latebroccoli system was harvested on August 31 and September 20, in 2009 and 2010, respectively.
Heads were hand harvested from the entire trial, and broccoli yield was estimated by recording
head counts and weights in a 3 m harvest row from three random plots per replicate. Broccoli
58
leaf, stem, and head samples were collected from a composite of three plants in each of three
random plots per replicate for dry matter and N content determination.
Treatments were applied 1 to 2 d after harvest with plots 9 m long by 3 m wide. The
control treatment representing the typical grower practice was the incorporation of broccoli crop
residue (CR-control), and was established by mechanically mulching then disking the broccoli
crop residue. The crop residue removal (CR-removal) treatment was established by handremoving all the above-ground biomass from the plots prior to disking. An oat (Avena sativa L.)
cover crop (CC-oat) was established by drilling seeds at a rate of 108 kg ha−1 after the broccoli
residue was mulched and incorporated. The OCA treatments were established by uniformly
hand-applying either wheat straw (OCA-straw, C:N ratio ≈ 65:1), yard waste (OCA-yard, C:N
ratio ≈ 56:1), or used cooking oil (OCA-oil, C:N ratio > 1000:1), at a fresh rate of 5 Mg ha−1
onto mulched broccoli crop residue (C:N ratio ≈ 11:1), followed by disking. The OCA-straw
material was obtained from Ridgetown Campus, the OCA-yard material was obtained from the
local municipality and consisted of brush material (twigs, branches, wood-chips, leaves), and
OCA-oil was obtained from a local restaurant.
In addition to samples at broccoli harvest, post-harvest soil samples in autumn were taken
for mineral N analysis from a composite of four soil cores per plot at depths of 0–30 and 30–60
cm on August 21, September 18, and October 22 for the early system in 2009, on August 20,
September 1, September 22, and October 25 for the early system in 2010, on October 26 for the
late system in 2009, and on October 6 and October 25 for the late system in 2010.
In the subsequent growing season, the entire trial area was cultivated and spring wheat
seed was drilled at 154 kg ha−1, without N fertilization. A broccoli residue incorporated treatment
59
with pre-plant urea applied at 103 kg N ha−1 (CRN-control) was also established as the treatment
representing the typical grower practice. Usual management practices were followed for spring
wheat production (OMAFRA 2009). Grain was mechanically harvested from the center area of 6
m by 1 m, and grain and straw were collected to assess the above-ground plant biomass (grain +
straw) and grain yield (Mg ha−1), and N content (kg N ha−1). Harvest occurred on August 3, 2010
and August 23, 2011. Soil was sampled, as described above, for mineral N analysis from depths
of 0–30, 30–60, and 60–90 cm at spring wheat planting and harvest.
3.3.1
Nitrogen Measurements
Plant N content (broccoli and spring wheat) was determined by dry combustion method
(Rutherford et al. 2008) using a LECO CN analyzer (Leco Corporation, St. Joseph, MI, USA),
following grinding with a 2 mm diameter mesh screen opening on a Wiley Mill (Thomas
Scientific, Swedesboro, NJ, USA). Soil NO3−-N and NH4+-N concentrations were quantified by
the KCl extraction method (Maynard et al. 2008). Briefly, 5 g of soil was extracted with 25 mL
of 2 M KCl, shaken for 30 min, filtered and analyzed colorimetrically on an AutoAnalyzer3
(SEAL Analytical Inc., Mequon, WI, USA) with high resolution digital colorimeter to quantify
NH4+-N (method G-102) and NO3−-N (method G-200). Soil mineral N was the sum of NO3−-N
and NH4+-N and expressed as kg N ha−1 based on soil bulk density.
3.3.2
Economic Analysis
The economic analysis was conducted through a comparison of spring wheat profit
margins for each treatment. Profit margins were calculated by subtracting costs that varied across
post-harvest treatments from revenues. Revenues were calculated based on spring wheat yields
60
from each treatment and the average spring wheat price in Ontario between 2010 and 2011, as
reported by the Ontario Ministry of Agriculture Food and Rural Affairs (OMAFRA 2012b).
Costs that varied included those associated with mulching residue (all treatments except the CRremoval treatment), residue removal, pre-plant N fertilizer for spring wheat crop, seeding cover
crop, and applying the OCA as listed in the OMAFRA’s Custom Rate Survey (OMAFRA
2009b). Costs associated with the OCA included transportation and application as well as baling
wheat straw. Some assumptions were made regarding the equipment and the associated rates that
would be used to conduct the required activities on a large field scale. For example, application
costs were based on the rates for manure spreading, while transportation costs were based on
trucking rates for wheat straw, yard waste, used cooking oil, and spring wheat grain (OMAFRA
2009b). In addition, the CR-removal method costs were based on custom rates for forage
harvesting (OMAFRA 2009b). Other costs which factored into the profit margin calculation
included the fertilizer cost, which was based on the average spring price reported in surveys of
retail outlets across Ontario between 2009 and 2011 (Anonymous 2011) and the oat seed cost,
which was based on prices provided by seed retailers in southern Ontario. All other costs were
assumed to be constant across treatments; thus, they were not accounted for in the profit margin
calculation.
3.3.3
Statistical Analysis
Because the analyses of variance (with fixed effects of treatment, system, year, and
respective interactions, and a random effect of block (system)) found significant year x system
effects (P<0.05) in datasets for SMN in autumn, spring, and spring wheat harvest, a separate
analysis of variance for each year and each early- or late-harvested system was necessary (with a
61
fixed effect of treatment and a random effect of block). Other than the late-broccoli-system in
2009 (which had only one sample date), the autumn SMN repeated measures had no treatment by
sample day effect (P>0.05) in each system in each year, thus the average of each repeated
measurement was investigated. A multiple means comparison to the CR-control and CRNcontrol (typical practices) with a Dunnett-Hsu adjustment was applied to each dataset. All
significant differences were set at P<0.05.
3.4 RESULTS AND DISCUSSION
3.4.1
Broccoli Harvest
Broccoli yields varied by system and year, but not by system × year. The early-broccoli
system had fresh yields of 22.2 and 22.9 Mg ha−1 and late-broccoli yielded 25.0 and 36.2 Mg
ha−1, in 2009 and 2010, respectively. Similar yields, 16 to 35 Mg ha−1, have been reported in the
literature (Zebarth et al. 1995; Everaarts and De Willigen 1999; Thompson et al. 2002; Yoldas et
al. 2008; Bakker et al. 2009b). The lower yield in the early-broccoli compared to the latebroccoli system agrees with previous research, which also found lower total and head plant mass
in summer harvested broccoli compared to autumn harvest (Gutezeit 2004), likely due to
different environmental conditions during the broccoli growth periods. The crop residue (leaves
+ stems) fresh weight ranged from 50 to 75 Mg ha−1, similar to broccoli and cauliflower crop
residue rates in earlier studies (Chaves et al. 2007a; Congreves et al. 2013b). The early-broccoli
crop residue contained 202 and 247 kg N ha−1, while the late-system residue had 212 and 207 kg
N ha−1, in 2009 and 2010, respectively. Similarly, previous research observed above-ground
broccoli N accumulation of 96 to 465 kg N ha−1, with fertilizer N application rates up to 500 kg
62
N ha−1 (Zebarth et al. 1995; Everaarts and De Willigen 1999a; Bowen et al. 1999; Thompson et
al. 2002; Bakker et al. 2009).
At broccoli harvest, 0–30 cm SMN ranged from 68 to 168 kg N ha−1, yet greater
quantities of N (207 to 247 kg N ha−1) were in the broccoli crop residue. Thus, between 265 and
415 kg N ha−1 remained in the field after harvest, which was consistent with other research
(Zebarth et al. 1995; Bakker et al. 2009a). Clearly, strategies that minimize N losses during the
post-harvest season would be valuable.
3.4.2
Soil Mineral Nitrogen in Autumn
Soil mineral N data were investigated because both NH4+-N (due to nitrification) and
NO3−-N are susceptible to losses. The treatment effect on soil N was similar for NO3−-N and
SMN concentrations, but the soil NH4+-N was largely unaffected by treatment. Thus, the SMN
results were presented. The 0–30 and the 30–60 cm depths were analyzed separately for the
autumn dataset to assess the downward movement and leaching potential of SMN in autumn.
The year variation in autumn SMN between 2009 and 2010 could be a residual N effect
attributed to the type of crop grown the year prior to broccoli production, which was corn and
soybean, respectively. Additionally, the higher autumn SMN variation in the early- compared to
the late-broccoli systems may be due to higher temperatures during the early autumn (August to
November) which may have permitted more decomposition and resulted in a greater N
mineralization, compared to cooler temperatures after the late-broccoli system (September to
November).
63
In the CR-control, autumn 0–30 cm SMN concentration ranged from 118 to 368 kg N
ha−1 (Figure 3.1) and SMN was affected by treatment. After the early-broccoli system and
relative to the CR-control, the 0–30 cm SMN concentrations were reduced by the OCA
treatments of OCA-straw in 2009 and 2010, OCA-yard in 2010 but not in 2009, and OCA-oil in
2009 and 2010 (Figure 3.1).
64
*
*
*
*
* * *
*
*
*
*
*
+
+
+++
+
+
+
+
+
+
+
Figure 3.1 Soil mineral N concentrations in the autumn, spring, and summer after the 2009 and
2010 early- and late-broccoli harvest treatments. Symbols denote a significant difference (P<0.05)
65
compared to the crop residue control * or the crop residue with pre-plant N control + (at wheat
harvest) based on a multiple means comparison with a Dunnett-Hsu adjustment. The se values
represent the standard error of the mean.
After the late-broccoli system, the only treatment which reduced SMN compared to the
CR-control was the OCA-oil in both 2009 and 2010 (Figure 3.1). The CC-oat treatment did not
have different 0–30 cm SMN concentrations compared to the CR-control (Figure 3.1). The CRremoval reduced the 0–30 SMN levels relative to the control in the autumn after early-broccoli
system, but not after the late-system in both years (Figure 3.1). No treatment differences were
found in the 30–60 cm depth (data not shown), with SMN concentrations ranging from 17 to 51
kg N ha−1 in the CR-control.
Due to the lack of effect on SMN by the CC-oat compared to the CR-control (Figure 3.1,
Table 3.3), it is suggested that oat cover crops may not reduce the potential for N losses after
broccoli harvest. Conversely, the establishment of an oat cover crop after green pea production
reduced autumn SMN concentrations in southwestern Ontario (O’Reilly et al. 2012). It is
possible that the CC-oat had low N uptake compared to the plant available N in the soil after
broccoli harvest, considering that 94 to 210 kg N ha−1 remained as available N at green pea
harvest (O’Reilly et al. 2012) while 265 to 415 kg N ha−1 remained at broccoli harvest.
Despite the removal of the N rich crop residue (CR-removal), the 0–30 cm SMN
concentrations were only reduced in the autumn after the early-broccoli and not after the latebroccoli system (Figure 3.1). The difference in CR-removal effect between systems was likely a
reflection of the cooler temperatures and slower N mineralization of the crop residue in the latesystem. Regardless, high autumn SMN concentrations (89 to 227 kg N ha−1) remained in the
66
field in the CR-removal treatment (Figure 3.1). It is therefore suggested that the soil and/or
below-ground crop residue provided substantial quantities of N during the post-harvest period.
Thus, best management practices that minimize N losses after broccoli production would be
beneficial.
Nitrogen immobilization is related to the biochemical composition of the decomposing
substrate, and is positively associated with a high C:N ratio, lignin, and polyphenol content
(Cabrera et al. 2005). Incorporation of easily decomposable, high C:N ratio materials generally
causes a rapid increase in microbial biomass and consequently SMN depletion as N is
assimilated into microbial cells. Soil amendments such as yard waste, wheat straw, and used
cooking oil/oily food waste have previously demonstrated N immobilization (Congreves et al.
2013b; Jawson and Elliot 1986; Amlinger et al. 2003; Plante and Voroney 1998; Rashid and
Voroney 2003).
The OCA-yard reduced SMN compared to the CR-control only after the early-broccoli
system in 2010 (Figure 3.1). The 2009 yard waste was composed of notably larger particles than
that of 2010, thus it is possible that decomposition took longer in 2009 and had less influence on
microbial N immobilization. More recalcitrant substrates, such as the lignin-containing wood
pieces of the OCA-yard, can have more limited decomposition at low temperatures than that of
easily decomposed and more labile material (Nicolardot et al. 1994). It is likely that C
decomposition of the OCA-yard was generally low, and consequently little microbial N
immobilization occurred. Perhaps if the OCA-yard material was finely chopped, greater
decomposition may occur. As opposed to the present study, previous laboratory research showed
N immobilization by incorporating yard waste with broccoli crop residue (Congreves et al.
67
2013b), and field research has found that green waste compost mixed with cauliflower crop
residue immobilized approximately 42 kg N ha−1 within the first month after incorporation
(Chaves et al. 2007a). Yet, in agreement with the present study, green waste composts have not
resulted in N immobilization during autumn after the incorporation with cauliflower or leek
residues in a two-year study (Chaves et al. 2007b). Thus, the composition and substrate size of
OCA-yard greatly influences N immobilization and its applicability as a better management
practice for minimizing N losses in the autumn after broccoli harvest.
Although OCA-straw reduced SMN after the early-broccoli system, it must be noted that
some wheat seed germinated and established a cover crop during both years. Thus, the reduction
in autumn SMN may be a reflection of cover crop N uptake as well as microbial N assimilation.
Conversely, it appeared that the OCA-straw treatment after the late-broccoli system did not
sufficiently lower SMN, compared to the CR-control, to reduce the potential for soil N losses in
the autumn. The difference in effect between systems was likely due to cooler temperatures and
slower decomposition (or cover crop uptake) after late-broccoli. In comparison to previous
laboratory research, it was demonstrated that wheat straw incorporated with broccoli crop
residue could significantly lower SMN via N immobilization, (relative to incorporating crop
residue alone) after 8 weeks of incubation (Congreves et al. 2013b). Previously, cereal straw
immobilized 35 kg N ha−1 within the first month after incorporation with cauliflower crop
residues in the field (Chaves et al. 2007a). It has been estimated that straw incorporation can
result in the net N immobilization of 64 kg N ha−1 after two months (Garnier et al. 2003), or 39
to 44 kg N ha−1 after one year (Mary et al. 1996), and reduce the amount of NO3−-N leached by
27% after a year (Garnier et al. 2003). The current study has suggested that the OCA-straw
68
treatment after the early-broccoli system can reduce SMN concentrations by 57 to 96 kg N ha−1,
which could otherwise be lost during autumn and over winter.
The OCA-oil treatment resulted in consistent N immobilization after both early- or latebroccoli systems (Figure 3.1). Compared to the typical grower practice (CR-control), the OCAoil treatment had 53 to 112 kg ha−1 less SMN, thus 30% to 50% less SMN could be available for
losses from the top 30 cm soil layer during the autumn after broccoli harvest. This finding was
consistent with previous field research, which found that oily food waste application in autumn
reduced soil NO3−-N by immobilization and lowered the potential for N losses by 47 to 56 kg N
ha−1 in the top 60 cm of soil (Rashid and Voroney 2003). Furthermore, a previous laboratory
study suggested that OCA-oil immobilized more SMN than OCA-straw or OCA-yard, when
incorporated with broccoli crop residue (Congreves et al. 2013b). The rate of used cooking oil
decomposition was synchronous with that of broccoli crop residue, whereas yard waste or wheat
straw decomposed slower than broccoli crop residue or used cooking oil (Congreves et al.
2013b). Because decomposition of more recalcitrant substances is more limited at low
temperatures than that of easily decomposed material (Nicolardot et al. 1994), the OCA-oil is
likely the most promising material to reduce the potential for soil N losses due to its readily
decomposable and labile matter. Thus, OCA-oil may be the most suitable amendment tested,
because broccoli can be harvested anytime from early August to late October in southern
Ontario.
3.4.3
Soil Mineral Nitrogen in the Subsequent Spring and Summer
The 0–90 cm profile was investigated to assess the quantity of SMN in the soil depth
accessible for the crop, at spring wheat pre-plant and harvest. Prior to spring wheat planting, the
69
SMN concentrations were generally not affected by the post-broccoli-treatments (Figure 3.1).
Because the 2011 spring had a one and a half times higher precipitation (512 mm from February
to May) compared to 2010 (343 mm from February to May), it is possible that SMN was
subjected to different mechanisms of concentration reduction depending on the year. In the CRcontrol, NO3−-N leaching may have occurred to a greater extent in 2011, thereby lowering spring
SMN concentrations. Although NO3−-N leaching could have also occurred in the OCA
treatments, the amendments may have immobilized N in the spring, also lowering SMN
concentrations. Additionally, the condition of high soil water content combined with the
presence of a readily decomposable C source and SMN, denitrification could have been favored.
Given the possibility of different mechanisms of SMN concentration reduction across treatments,
further investigation is required. Future research should focus on 15N labeled crop residue to
investigate the influence of the OCA treatments on the fate of the crop residue-derived N.
At spring wheat harvest, 0–90 cm SMN results indicated that the post-broccoli-harvest
treatments did not lower SMN, compared to the CR-control (Figure 3.1). However, the
application of N fertilizer at spring wheat pre-plant (CRN-control) left a larger quantity of SMN
at harvest compared to most other treatments (Figure 3.1).
3.4.4
Spring Wheat Production
Overall, the 2011 spring wheat grain contained 49 kg N ha−1 less N and had 1.6 Mg ha−1
less yield than 2010 (Figures 3.2 and 3.3). Early- and late-broccoli systems did not have different
spring wheat yield or N contents. The yield ranged from 0.9 to 3.4 Mg ha−1 (Figure 3.3), similar
to the average Ontario spring wheat yields of 3.5 Mg ha−1 in 2010 and 2011 (OMAFRA 2012b).
The N harvest indices ranged from 55% to 90%, and did not vary by treatment (data not shown).
70
It must be noted that spring wheat plant N content, grain N content, plant biomass, or
grain yield were never different between the CRN-control and the CR-control (Figure 3.2, Figure
3.3). It therefore appears that sufficient soil N was available for crop production subsequent to
broccoli crop residue incorporation, regardless of pre-plant fertilizer application for spring
wheat. Thus, growers may be able to reduce N fertilizer applications to spring wheat planting,
following broccoli production because the mineralization of crop residue may provide sufficient
SMN.
71
Spring Wheat Plant N (kg N ha-1)
150
100
+
*
50
+
*
0
early-broccoli 2009 late-broccoli 2009 early-broccoli 2010 late-broccoli 2010
se=6.5
se=12
se=10
se=5.0
2010 planting
2011 planting
Spring Wheat Grain N (kg N ha-1)
150
100
50
+
*
*
+
*
0
early-broccoli 2009 late-broccoli 2009 early-broccoli 2010 late-broccoli 2010
se=5.2
se=9.0
se=5.3
se=3.5
2010 planting
2011 planting
Figure 3.2 The N content (kg N ha−1) of spring wheat plant biomass (grain + straw) and grain in
2010 and 2011, as affected by the previous years’ treatments in the early- and late-broccoli systems.
Symbols denote a significant difference (P<0.05) compared to the crop residue control * or the crop
residue with pre-plant N control +, based on a multiple means comparison with a Dunnett-Hsu
adjustment. The se values represent the standard error of the mean.
72
Spring Wheat Plant Biomass (Mg ha-1)
10
8
6
+
+
4
*
+
*
2
0
early-broccoli 2009 late-broccoli 2009 early-broccoli 2010 late-broccoli 2010
se=0.4
se=0.5
se=0.5
se=0.2
2010 planting
2011 planting
Spring Wheat Grain Yield (Mg ha-1)
10
8
6
4
+
*
+
*
2
+
*
+
*
+
*
0
early-broccoli 2009 late-broccoli 2009 early-broccoli 2010 late-broccoli 2010
se=0.2
se=0.2
se=0.2
se=0.1
2010 planting
2011 planting
Figure 3.3 The spring wheat plant biomass (grain + straw) and grain yield (Mg ha−1) in 2010 and
2011, as affected by the previous years’ treatments in the early- and late-broccoli systems. Symbols
denote a significant difference (P < 0.05) compared to the crop residue control * or the crop residue
with pre-plant N control +, based on a multiple means comparison with a Dunnett-Hsu adjustment.
The se values represent the standard error of the mean.
73
The treatments which indicated a detrimental effect on spring wheat production were the
OCA-straw and CC-oat, based on a lower spring wheat plant N content, grain N content, plant
biomass, and grain yield, compared to the CR-control or CRN-control (Figure 3.2, Figure 3.3).
Also, the CR-removal treatment indicated a reduction in spring wheat plant biomass and grain
yield compared to the CR-control or CRN-control in 2010 (Figure 3.2, Figure 3.3). Conversely,
the OCA-yard and OCA-oil treatments did not have different plant N content, grain N content,
plant biomass, and grain yield, compared to the CR-control or CRN-control (Figure 3.2, Figure
3.3). Therefore, it appears that the OCA-yard or OCA-oil treatments after broccoli harvest did
not negatively impact the subsequent spring wheat production, but the OCA-straw, CC-oat, and
CC-removal treatments resulted in spring wheat yield penalties.
It is possible that N supply was sufficient for spring wheat production in the CR-control,
CRN-control, OCA-yard, and OCA-oil treatments. To optimize N use efficiency, early-season N
availability has been recommended to achieve a desired spring wheat yield goal, rather than
late-season N availability (Cassman et al. 1992). If early-season SMN levels were limiting for
plant production, decreased vegetative dry matter accumulation and grain yield may occur
(Cassman et al. 1992). Thus, the rate and timing of OCA and its decomposition is crucial for
determining N dynamics. The grain yield results suggested that N supply was sufficient for the
spring wheat growing season after the OCA-yard and OCA-oil, but perhaps not the OCA-straw
(Figure 3.3). Likewise, the CC-oat and CR-removal treatments may not have had sufficient
available N (Figure 3.3). Thus, N fertilizer applications may be required to maintain the spring
wheat yield after OCA-straw, CC-oat, and CR-removal practices.
74
The lower spring wheat grain yield compared to the CR- or CRN-control in 2011 after
the OCA-straw or CC-oat treatment (Figure 3.3) may have been an allelopathic result, because
straw mulch often reduces subsequent wheat yields (Wu et al. 2001). Considering that wheat
seed in the OCA-straw treatment germinated and established a cover crop after broccoli harvest,
the continuous cereal cropping from autumn to the subsequent summer may have accumulated
phytotoxins or pathogens in the soil (Wu et al. 2001), which lowered the grain yield in 2011.
Variation in environmental conditions between years likely contributed to the severity of
phytotoxic or pathogenic factors. It is also possible that plant available N concentrations were
not sufficient for optimal spring wheat yield in the OCA-straw and CC-oat treatments. In a
previous study, which incorporated cereal straw with cauliflower crop residues, a pattern of
autumn N immobilization was followed by N re-mineralization in the following spring (Chaves
et al. 2007a). However, a different study found no apparent re-mineralization even one year after
straw incorporation in the field (Mary et al. 1996).
The lack of effect on spring wheat harvest parameters from the OCA-yard treatment
(Figure 3.2, Figure 3.3) may be related to the limited effect on SMN after broccoli harvest in
autumn (Figure 3.1). Similarly, limited autumn N immobilization and no consistent N remineralization were found following the autumn incorporation of green waste compost or
sawdust with cauliflower or leek crop residues (Chaves et al. 2007a; 2007b). It was suggested
that rye, leek, or lettuce production may not be negatively affected following autumn cauliflower
crop residue incorporation with compost or straw amendment because plant N uptake or dry
matter accumulation were generally similar between amendment and crop residue alone
treatments (Chaves et al. 2007a), which was similar to the present OCA-yard results.
75
Considering that autumn N immobilization occurred in the OCA-oil treatment (Figure
3.1), yet spring wheat harvest parameters were generally not affected (Figure 3.2, Figure 3.3), it
is possible that N losses may be reduced during a period of high risk for losses, without
negatively affecting the subsequent crop. It is therefore suggested that the OCA-oil after broccoli
harvest may be a better N management practice than the typical practice of the CR- or CRNcontrol. Although N was immobilized during the autumn after broccoli harvest, it appeared that
OCA-oil did not conserve more N in the soil compared to the CR-control for subsequent spring
wheat use (Figure 3.1). A reason may be that little or no NO3−-N leaching occurred in the CRcontrol treatment; the soil textures with 24% to 25% clay (loam or sandy clay loam) (Table 3.1)
would support a slower rate of NO3−-N leaching than sandier soils. It is possible that some of the
N immobilized by the OCA-oil re-mineralized and was lost (via denitrification or leaching) prior
to wheat uptake. Alternatively, the soil may have provided substantial quantities of plant
available N regardless of the treatment, as suggested earlier. Thus, future experiments should
investigate the fate of broccoli crop residue-derived N as influenced by the used cooking oil in N
limited soil, sandy soil, or trace the crop residue-derived N into the subsequent crop via 15N
studies.
A pattern of N immobilization followed by re-mineralization with used cooking oil or
“fat oil grease” amendments has been found in previous laboratory (Congreves et al. 2013b) and
field (Rashid and Voroney 2003) research. In the present study, N immobilized in the OCA-oil
after broccoli harvest in autumn could have re-mineralized for the subsequent spring wheat plant
uptake. Net N mineralization has also been observed in the spring following an autumn
application of oily food waste (Rashid and Voroney 2003). Previous research has found corn
yields to be similar between un-amended treatments and autumn amendments of “fat oil grease”
76
(Rashid and Voroney 2004). Based on the maximum economic rate of N applied to the corn
crop, it was found that N availability to corn was not affected by autumn application of “fat oil
grease” (Rashid and Voroney 2004). The authors suggested that sufficient time was probably
available for decomposition of the C material when it was applied in autumn (Rashid and
Voroney 2004). Also, researchers have found little concern for detrimental accumulations of oily
food waste and observed that the waste can promote water stable aggregation (Plante and
Voroney 1998). However, the spring application of “fat oil grease” prior to corn production
resulted in net N immobilization during the spring (Rashid and Voroney 2003), reduced corn
yields up to 23%, and the additional requirement of 60 kg N ha−1 to offset corn yield declines
(Rashid and Voroney 2004). Yet, “fat oil grease” or OCA-oil applied in the autumn did affect the
following corn yield (Rashid and Voroney 2003) or spring wheat yield.
3.4.5
Economic Analysis
The potential environmental benefit of reduced N losses must be considered in
combination with economic outcomes. Although the OCA-straw and CR-removal treatments
showed some potential for reducing autumn N losses after early-broccoli systems (Figure 3.1), it
not only resulted in reduced spring wheat production parameters (Figure 3.2, Figure 3.3), but
also reduced spring wheat profit margins (Table 3.3), relative to the CR- or CRN-control. Even
without detrimental effects on spring wheat production parameters, the OCA-oil treatment
reduced profit margins compared to the CR- or CRN-control (Figure 3.2, Figure 3.3, Table 3.3).
However, of all the treatments which demonstrated a potential for reduced autumn N losses,
(after early-broccoli by OCA-wheat, CR-removal treatments, or after both early- and latebroccoli by the OCA-oil), the OCA-oil treatment may be the least likely to lower spring wheat
77
profit margins compared to CRN-control (Table 3.3). Furthermore, the economic results suggest
that pre-plant application of N fertilizer for spring wheat was not necessary due to the similar
profit margins between the CR-control and CRN-control (Table 3.3). However, soil N was not
limited at Ridgetown. Thus, the question remains whether re-mineralization of immobilized N by
OCA-oil could result in an economic advantage if soil N was limited and re-mineralized N
contributed to spring wheat yield.
Table 3.3 The effect of post-broccoli-harvest treatments on 2010 and 2011 spring wheat profit
margins ($ ha−1) subsequent to the 2009 and 2010 early- and late-harvested broccoli.
Post-broccoli-harvest treatment
Broccoli production system
2009
2010
lateearlylatebroccoli
broccoli
broccoli
Spring wheat profit margins ($ ha−1)
2010
2011
617
263
306
earlybroccoli
Crop residue control
586
Crop residue with pre-plant N
527
511
228
229
Crop residue removal
429 *+
422 *
75 *+
84 *+
Oat cover crop
394 *+
440 *
144 *+
131 *+
Wheat straw
434 *+
352 *+
70 *+
24 *+
Yard waste
515
579
205
228
Used cooking oil
508 *
506
235
197 *
Standard error of mean (se)
31.7
39.5
34.2
25.1
control
Symbols denote a significant difference (P<0.05) compared to the crop residue control * or to the crop
residue with pre-plant N control +, based on a multiple means comparison with a Dunnett-Hsu
adjustment.
78
It has previously been indicated that trade-offs exist between environmental and
economic benefits, because practices which reduced NO3−-N losses caused lower economic
returns (Saleh et al. 2007). Also, vegetable production practices, which reduced NO3−-N
leaching, did not always coincide with those that generated optimal economic outcomes
(Thompson et al. 2000). Although a prior study on “fat oil grease” amendments did not
specifically determine the economic impact (Rashid and Voroney 2004), the impact can be
estimated based on the observed effects on the following corn yield and N requirements. The
reported mean control plot yields (Rashid and Voroney 2004) and the average Ontario prices in
1996 and 1997 for corn and urea (Anonymous 1997) suggested that the use of “fat oil grease”
reduced the revenue by as much as $290 ha−1 and increased the input costs by about $52 ha−1.
The reduced profit margins of autumn applied OCA in corn was similar to the present study in a
broccoli-spring wheat rotation. Thus, best management practices that have potential
environmental benefit may also have an economic cost for growers.
3.5 CONCLUSION
Better N management practices are necessary after cole crop production due to the high
SMN concentration and the risk for N losses in the post-harvest season. Although the OCA-straw
demonstrated reduced autumn SMN concentrations after the early-broccoli system, it is not
recommended if spring wheat is the following crop due to yield reductions. Also, the OCA-yard
showed inconsistent potential for reduced autumn soil N losses. However, the practice of OCAoil is recommended due to the reduced potential for autumn N losses via N immobilization after
both early- and late-broccoli systems, without the subsequent spring wheat yield being
detrimentally affected. This field study was consistent with previous incubation research, which
79
found that OCA-oil demonstrated the most promise for the potential reduction in soil N losses
after broccoli harvest (Congreves et al. 2013b). Despite the environmental benefits of potentially
reduced N losses by OCA, economic costs were associated. Thus, growers must evaluate the
environmental vs. economic benefits of OCA-oil compared to the typical practice.
80
4
AMENDING SOIL WITH USED COOKING OIL TO REDUCE NITROGEN LOSSES
AFTER COLE CROP HARVEST: A 15N STUDY.
4.1 ABSTRACT
After cole crop (Brassicaceae) harvest, over 400 kg N ha-1 may remain in the field as crop
residues and soil mineral N. Thus, methods to reduce potential post-harvest N losses are needed.
The fate of 15N enriched crop residue-derived N (CRN) and residual fertilizer or root biomass N
(RN) was studied from broccoli (Brassica olecerea var italica L.) harvest (Aug and Sept 2011)
to spring wheat (Triticum durum L.) harvest (Jul 2012), with and without the C amendment of
used cooking oil. Urea with 5% 15N excess was incorporated (342 kg N ha-1) in micro-plots to
label broccoli. At harvest enriched and natural abundance 15N above-ground broccoli residues
were exchanged and incorporated with or without used cooking oil (5 Mg ha-1). The RN was
mostly organic N, not influenced by amendment, and resistant to post-harvest losses. With used
cooking oil vs. without, soil mineral CRN was reduced by 19 kg ha-1 and microbial biomass
CRN was increased by 21 kg ha-1 (P<0.05) two weeks after broccoli harvest, indicating
immobilization of CRN and reduced potential N losses. At spring wheat harvest, the used
cooking oil treatment had greater total, organic, and mineral soil CRN compared to the control of
no oil by 44, 43 (P<0.1), and 0.75 (P<0.05) kg ha-1, respectively. The amendment increased
recovery of CRN in soil total N, organic N, and mineral N by 190 to 209% over the year, yet it
did not affect spring wheat yields or plant N uptake. It is hypothesized that used cooking oil
facilitated the incorporation of CRN into stable organic compounds which were less susceptible
to losses. It is recommended that growers apply used cooking oil at cole crop harvest to
minimize potential above-ground crop residue-N losses and to increase the soil organic N
fraction.
81
4.1 INTRODUCTION
The development of sustainable agricultural practices focused on post-harvest soil N
management is necessary for cole crop production because up to 415 kg N ha-1 may remain in
the field as soil mineral N and crop residue N (Zebarth 1995; Bakker et al. 2009a; Congreves et
al. 2013a) which rapidly mineralize (De Neve and Hofman 1996; Congreves et al. 2013b). Cole
crop residue-derived N may readily leach or contribute to greenhouse gas emissions (De Neve
and Hofman 1998; Velthof et al. 2002) during the post-harvest period, due to the annual water
budget in Ontario (Fallow et al. 2003) and elsewhere with similar climates.
Previous field (Rashid and Voroney 2003; Congreves et al. 2013a) and laboratory (Plante
and Voroney 1998; Congreves et al. 2013b) studies have demonstrated potential for reducing soil
N losses with organic C amendments such as oily food waste or used cooking oil. Under
controlled conditions and incorporated with above-ground broccoli crop residue, an amendment
of used cooking oil resulted in net N immobilization of 86 mg kg-1 (Congreves et al. 2013b). In
the field after broccoli harvest, used cooking oil reduced soil mineral N by 53 to 112 kg N ha-1 in
autumn without lowering yields or altering plant N uptake of the subsequent spring wheat crop
(Congreves et al. 2013a). Thus, used cooking oil appears to be an effective best management
practice to minimize N losses.
However, the below-ground crop residue, indigenous soil N mineralization, and/or
residual fertilizer N may also provide a source of soil mineral N (>100 kg N ha-1) which could be
susceptible to losses after cole crop harvest (Zebarth et al. 1995; Congreves et al. 2013a).
Previous research has not yet determined if the residual N from fertilizer or vegetable root
biomass has the same risk for losses as does the above-ground crop residue N. Thus, the quantity
82
of N-derived from broccoli crop residue must be separated from that of soil N to accurately
assess the fate of above-ground crop residue-N. Furthermore, mineralization and immobilization
dynamics of vegetable root residues are largely unknown.
Although Congreves et al. (2013a; 2013b) assessed the soil mineral N dynamics after
cole crop harvest with used cooking oil amendments, further analyses of total N, microbial
biomass N, and organic N are needed to better understand how the amendment influences the
fate of N. Thus, questions remain: how much above-ground and below-ground crop residue
derived-N is retained in the soil N pools after amending soil with used cooking oil, and how
much of the crop residue-N becomes available for plant growth during the subsequent growing
season? Measurements of N-derived from crop residues can be done using 15N labelling
techniques, where the 15N labelled residue is applied to an unlabelled soil-plant system. This
method is most suitable to trace the fate of N from crop residue into different soil N pools and
subsequent plants (Douxchamps et al. 2011).
The objective of this field study was to evaluate the fate of broccoli crop residue-derived
N after harvest and in a subsequent spring wheat crop, with and without applying used cooking
oil. This research could contribute to the development of best management practices for
sustainable cole crop production by minimizing potential soil N losses and by furthering the
understanding of post-harvest N dynamics.
83
4.2 MATERIALS AND METHODS
Field Site 2011-2012
The experimental site located at the University of Guelph, Ridgetown Campus is a
Brookston clay loam soil (Orthic Humic Gleysol, Canadian Soil Classification; mixed, mesic
Typic Argiaquoll, USDA Soil Taxonomy). The initial soil characteristics, evaluated by an
accredited laboratory (SGS Agri-Food Laboratories Ltd., Guelph, ON) according to Carter and
Gregorich (2008), included soil pH of 6.4 (1:1 v v-1 method), 46 g kg-1 organic matter (loss on
ignition), 37 mg kg-1 P (Olsen bicarbonate extraction), 3240 mg kg-1 Ca, 53 mg kg-1 K, 144 mg
kg-1 Mg (atomic absorption via ammonium acetate extraction), 18.7 cmol kg-1 cation exchange
capacity (CEC) (estimated based on ammonium acetate extraction and pH), soil texture of 53%
sand, 25% silt, 22% clay (hydrometer method), and 1.3 g cm-3 bulk density. Temperature and
precipitation data were obtained from the on-site weather station (Table 4.1).
84
Table 4.1 Monthly temperature (°C) and precipitation (mm) at Ridgetown, ON during the broccoli
- spring wheat crop rotation.
--------------------Temperature (°C) --------------------
---------Precipitation (mm) -------30 yr Mean
30 yr Mean
Mean
Min
Max
Max
Total
Total
(daily)
Jun 2011
20
19
13
24
86
75
33
Jul 2011
22
24
18
29
86
70
24
Aug 2011
21
20
14
26
93
70
18
Sept 2011
18
17
12
21
69
135
21
Oct 2011
11
10
5.8
15
75
79
33
Nov 2011
4.8
6.9
2.8
11
67
140
62
Dec 2011
-1.2
1.6
-1.6
4.8
61
86
20
Jan 2012
-3.7
-1.8
-5.8
2.2
54
55
16
Feb 2012
-2.4
-0.3
-3.5
2.9
60
32
13
Mar 2012
2.0
7.6
2.4
13
78
52
19
Apr 2012
8.3
7.0
0.9
13
75
32
12
May 2012
15
16
9.3
22
83
34
6.5
Jun 2012
20
23
15
23
86
45
-
Jul 2012
23
22
16
28
86
156
55
4.2.1
Experimental Design
The 15N tracer trial was located within a larger field trial, as described by Congreves et al.
(2013a), following the same timing of field activities (planting, fertilizing, and sampling) (±1 d).
The 15N trial had a randomized complete block design with four replications, two broccoli (c.v.
Ironman) systems: early (August) and late (September) harvest, and two treatments: a control
and a used cooking oil amendment.
85
Aluminum metal sheets (18 cm in height) were used to construct the frames for the 15N
mini-plots (1.5 by 0.6 m) and two frames were inserted 10 cm deep into the soil, 2 m apart and
within the centre of the field treatment plots (9 by 4.5 m) of both early and late broccoli systems.
Half of the total number of mini-plots was used to produce 15N enriched broccoli plants and the
rest were used for producing natural abundance 15N plants. Within mini-plots and prior to
transplanting broccoli, 5 atom% enriched 15N urea fertilizer (Cambridge Isotope Laboratories,
Cambridge, Massachusetts, USA) mixed with silica quartz sand (fertilizer: sand ratio of 1:2 by
weight) was uniformly spread by hand onto the soil surface and incorporated with a hand-trowel
to simulate disking to a depth of 15 cm. To produce natural abundance 15N broccoli, nonenriched urea was spread and incorporated in mini-plots as described above and in the
surrounding plot area. Both 15N enriched and natural abundance fertilizer corresponded to a rate
of 342 kg N ha-1.
Broccoli transplants were hand planted on June 1 for the early system and on July 4 for
the late system, in 2011. Each mini-plot contained four broccoli plants. Experimental set-up,
sample collection and processing always occurred first for the natural abundance 15N samples
and second for the enriched 15N samples to avoid 15N transfer and contamination.
At broccoli harvest (Aug 19 and Sept 22, 2011 for the early and late systems,
respectively) a composite sample of three broccoli plants (roots and shoots) per replicate were
collected to approximately 30 cm depth, separated into roots, stem, leaves, and head, and
collected from natural abundance and 15N enriched soil for N analysis. Rhizosphere soil samples
were collected by removing soil particles directly adhering to the roots. Also at broccoli harvest,
86
a composite soil sample was taken from four cores at the depths of 0-30 and 30-60 cm per
replicate, from natural abundance and 15N enriched soil for N analysis.
After broccoli harvest, the above-ground crop residue of four natural abundance and four
enriched 15N plants (stem and leaves, but no heads) were collected, weighed, chopped (to
approximately 1 cm2) with a mechanical leaf mulcher and the fresh weight of the mulched crop
residue was recorded. The above-ground crop residue of enriched and natural abundance 15N
plants were exchanged, thus splitting the 15N enrichment into two factors: (i) above-ground
broccoli crop residue-derived N (CRN) and (ii) residual fertilizer or broccoli root biomass N
(RN). Treatments included a control of incorporated crop residue with no amendment, and the
crop residue incorporation with used cooking oil (applied uniformly at 5 Mg ha-1 with a watering
can). The used cooking oil was canola and obtained from a local restaurant (Ridgetown,
Ontario). Soil within the mini-plots was homogeneously incorporated by hand with a shovel to
15 cm depth.
To follow the 15N tracer after broccoli, soil was sampled to 0-30 and 30-60 cm for natural
abundance and enriched 15N: two weeks after broccoli harvest (on Sept 5, 2011 for the early
broccoli system; Oct 10, 2011 for the late broccoli system), prior to soil freeze-up or snow on
Nov 6, 2011, prior to spring wheat seeding on Apr 13, 2012, and at spring wheat harvest on Jul
25, 2012. The core holes were plugged with wooden dowels (3.2 cm diameter) after autumn soil
sampling to minimize preferential flow overwinter; dowels were removed to prepare soil for
spring wheat plating in April.
Prior to spring wheat seeding on Apr 13, 2012, cultivation was simulated in the miniplots with a hand trowel to 15 cm depth. Eighteen g of spring wheat seed (which corresponded to
87
162 kg ha-1) was hand broadcast and incorporated to the surface 5 cm soil with a hand trowel.
Spring wheat was seeded with a drill in the surrounding field area. On July 25, 2012 spring
wheat was hand harvested by clipping biomass within the mini-plots at ground level and each
biomass sample was processed through a small combine which separated the grain and straw. To
minimize cross contamination, the combine was left running between each sample collection.
4.2.2
Nitrogen Measurements
For plant total N (TN (14N and 15N)) and 15N, fresh broccoli and spring wheat samples
were weighed for fresh weight, dried at 60°C, re-weighed for dry matter content determination,
finely ground with a 0.4 mm screen mesh opening on a Thomas-Wiley mini-mill (Thomas
Scientific, Swedesboro, NJ, USA) and weighed into 8 by 5 mm tin capsules (Elemental
Microanalysis, Okehampton, Devon, UK).
For soil total 14N and 15N (TN) and 15N, soil mineral TN and 15N analysis, soil samples
were kept frozen (-20°C) and thawed at room temperature for sample preparation. For microbial
biomass TN and 15N analysis, samples were immediately processed without freezing or drying.
However, microbial biomass N analysis was only performed on autumn soil samples. For soil
total TN and 15N analysis, subsamples were dried at 60°C for moisture determination, finely
ground and weighed into tin capsules. For soil mineral TN and 15N analysis, subsamples were
processed by the KCl extraction method (Maynard et al. 2008), followed by an acid diffusion
method adapted from Brooks et al. (1989). Briefly, 5 g of soil was extracted with 25 mL of 2 M
KCl, shaken for 30 min, filtered; 3 mL of extract was mixed with 40 mg of Devarda’s Alloy and
1 mL of 1 N NaOH inside a 20mL Nalgene vial. The vial was tightly sealed with rubber stopper
which had a steel hook that suspended a Whatman Grade 1 filter disk infused with 10 μL of 0.25
88
N KHSO4. The filter disks were collected after 24 hr and inserted into tin capsules. For microbial
biomass TN and 15N, the fresh soil subsamples were immediately processed by the chloroformfumigation-extraction method (Voroney et al. 2008), followed by K2S2O8 oxidation and
autoclaving (Cabrera and Beare 1993), and acid diffusion (as previously described, adapted from
Brooks et al. (1989)).To ensure validity, blank (KCl) and check (5 ppm NO3--N) samples were
included with each sampling and analysis step (i.e. extraction, acid diffusion, chloroformfumigation-extraction, oxidation, and isotope analysis).
All soil and plant samples were analysed for TN and 15N abundance using gaschromatography-mass spectrometry at the Stable Isotope Facilities, University of Saskatchewan,
Saskatoon, SK. Also, finely ground plant seed and soil samples with a mean atom % 15N of
0.3673 and 0.3684, respectively, were used as analytical references.
4.2.3
Calculations
All soil N quantities were expressed on an area basis (i.e. kg ha-1), based on soil bulk
density, N concentration and depth. Soil rhizosphere N quantities were expressed as kg ha-1,
based on planting density and assuming that the volume of rhizosphere soil had a 10 cm deep
volume, as suggested by Gregory (2006). All plant N quantities were expressed on an area basis
(i.e. kg ha-1) and based on dry matter weights. The N fertilizer uptake efficiency (NUE) was
calculated from the field plot broccoli samples as the difference in total aboveground plant N
content in the fertilized broccoli treatment from the zero N treatment divided by N fertilizer
applied and multiplied by 100.
89
The 15N enrichments were obtained by subtracting the 15N abundance of the respective
natural abundance sample from the 15N enriched sample and the following calculations
[equations 21 to 26] are similar to previous 15N research (Douxchamps et al. 2011).
The percent of N-derived from fertilizer at harvest (FN) and after harvest (RN) was
calculated as:
[21]
where the atom % 15N excess of the compartment was the 15N enrichment of the pool considered
(either broccoli plant part or soil N pool) and the atom % 15N of the fertilizer was the enriched
urea applied at pre-plant.
To trace the fate of crop residue-derived N after broccoli harvest, a weighted 15N excess
was used for CRN, calculated from plant parts of stem and leaves:
[22]
The percent N derived from the above-ground broccoli crop residue (CRN) was
calculated as:
[23]
where the compartment referred to the N pool (i.e. plant, soil 0-30 or 30-60 cm, total or mineral).
The amounts of N derived from the fertilizer (FN), above-ground crop residue (CRN),
and residual fertilizer or roots (RN) were calculated as:
90
[24]
The N recovery of fertilizer (FN), above-ground crop residue (CRN), and residual
fertilizer or roots (RN) was calculated as:
[25]
where N applied for FN or RN was the amount of pre-plant urea fertilizer N, but for CRN it was
the amount of crop residue N.
The microbial biomass TN or microbial biomass N-derived from the crop residue (CRN)
was calculated as follows:
[26]
where, fum and non-fum denotes the fumigated and non-fumigated sample, respectively, and a
value of 0.45 was applied as an extraction efficiency factor (KEN) (Jenkinson et al. 2004).
The soil organic N pool (FN, RN, CRN, or TN) was calculated as the difference of total
soil and mineral soil N.
4.2.4
Statistical Analysis
The analyses were conducted with PROC MIXED in SAS (SAS Institute, Inc. version
9.3, Cary, NC). The broccoli harvest data were subjected to an analysis of variance and Tukey’s
multiple means comparison and significant differences were noted at the probability level of
P<0.05.
91
To follow the15N tracer after broccoli harvest at each sample date, the RN, CRN, and TN
data were subjected to an analysis of variance which included fixed effects of treatment, system,
depth, all respective interactions, and a random effect of block(system) at P<0.05. If there was a
depth x treatment interaction, then the 0-30 and 30-60 cm soil depths were separately
investigated, otherwise the 0-30 and 30-60 cm data were summed and the 0-60 cm profile
assessed. The broccoli systems (early and late) were pooled if there was no treatment x system
interaction. Treatment differences with or without used cooking oil in each RN, CRN, and TN
pool were noted at P<0.001, 0.1, 0.05, and 0.1 with an LSD test.
The effect of used cooking oil was also expressed as a percent of the un-amended control
over time (months) for CRN total, mineral, and organic pools within the 0-60 cm profile, and
fitted to a non-linear regression via PROC NLIN for a first order exponential model y=A(1-expk(t)
) (y=percent of control, A=coefficient indicates potential CRN, k=rate constant, t=time in
months). The square of the correlation between the predicted and observed values (Efron’s
Pseudo R2) was calculated to assess the predictive strength of the non-linear regression.
For comparison to the 15N trial, the non-labelled field plot soil mineral N (sum of NO3--N
and NH4+-N) and soil NO3--N were assessed within the whole 0-60 cm profile at each sample
date. Thus, field plot soil mineral N, NO3--N, and spring wheat harvest data were subjected to an
analysis of variance with fixed effects of treatment, system, treatment x system and a random
effect of block (system). The multiple means comparison was based on a Dunnett-Hsu
adjustment, contrasted against the crop residue incorporated control and the crop residue
incorporated control with N fertilizer to spring wheat. Treatment differences were noted at
P<0.001, 0.01, 0.05, and 0.1.
92
4.3 RESULTS AND DISCUSSION
4.3.1 Fate of Fertilizer Derived Nitrogen at Broccoli Harvest
There was no mini-plot or system effect or interaction on broccoli yield (i.e. P>0.05).
The mini-plot frame appeared to have little influence on production because the broccoli plants
had a mean yield of 21.7 Mg ha-1 with 60.0 Mg ha-1 of fresh crop residue containing 287 kg ha-1
of TN (Table 4.2), which was comparable to yields (18.9 Mg ha-1), crop residue quantities (62.0
Mg ha-1), and crop residue N (205 kg N ha-1) from the field plot trial. Similar yields (16 to 36 Mg
ha-1), fresh crop residue quantities (50 to 75 Mg ha-1), and crop residue N (202 to 247 kg N ha-1)
have been previously found for cole crop production and/or residue incorporation studies
(Zebarth et al. 1995; Everaarts and De Willigen 1999a; Thompson et al. 2002; Chaves et al.
2007, Yoldas et al. 2008; Bakker et al. 2009b; Congreves et al. 2013a). Also, Ontario cole crop
yields in 2011 ranged from 7.5 to 28 Mg ha-1 for broccoli, cauliflower, and cabbage (OMAFRA,
2011). Therefore, the current experimental broccoli production was representative of the region
and comparable to other studies.
At harvest, the broccoli plants (head, leaves, stem, and root) recovered 54.3% of the FN
applied (Table 4.2). Of the root material collected, roots recovered only 2.75% FN (Table 4.2).
The above-ground crop residue (leaves and stem) had 125 kg ha-1 FN and recovered 36.6% FN
(Table 4.2). Thus, the majority (70.1%) of FN in the above-ground plant biomass was returned to
the soil as crop residue. Similarly, previous 15N research found 46% of fertilizer-derived N was
recovered in cauliflower biomass after a potato-cauliflower rotation, of which 52% was returned
to the soil as cauliflower crop residues (Akkal-Corfini et al. 2010). Other tracer studies have
found lower FN recoveries in above-ground plant biomass at cabbage harvest, ranging from 16
93
to 20% (Nissen et al. 1999) and 30 to 38% (Choi et al. 2004). Typically, recovery of fertilizer N
in roots has not been reported in the literature and the aforementioned studies did not include
root N. Based on this 15N study, a shoot to root FN ratio of 19:1 may be used to estimate total
plant uptake and fertilizer N recovery in broccoli.
Table 4.2 The fate of fertilizer-derived N at harvest of early (Aug) and late (Sept) broccoli
production in 2011†.
Fertilizer-derived N
Fertilizer-derived N
Total N
(kg ha-1)
(% recovery)
(kg N ha-1)
Early and Late
Early and Late
Early and Late
Head
50.8 B
14.9 B
120 B
Leaf
91.6 A
26.9 A
209 A
Stem
33.1 B
9.74 B
78.3 B
Root
9.35 C
2.75 C
21.0 C
5.77
1.69
13.5
Early and Late
Early and Late
Early
Late
Total 0-30 cm
52.2 ABC
15.3 ABC
6996 A
6390 A
Total 30-60 cm
67.8 AB
19.8 AB
3500 B
6920 A
Total rhizosphere
99.3 A
29.0 A
8236 A
10061 A
Mineral 0-30 cm
10.3 BC
3.02 BC
46.9 C
30.3 B
Mineral 30-60 cm
8.35 BC
2.44 BC
45.1 C
25.1 B
5.10 C
1.49 C
58.7 C
38.4 B
Organic 0-30 cm
41.8 ABC
12.2ABC
6949 A
6360 A
Organic 30-60 cm
59.5 ABC
17.4AB
3455 B
6895 A
94.2 A
27.5A
8177A
10023 A
13.1
3.82
322.63
871.80
N Pool
Plant
Standard error
Soil
Mineral rhizosphere
Organic rhizosphere
Standard error
† Means followed by different letters within columns for plant or soil pool were significantly different at
P<0.05, based on a Tukey’s multiple means comparison.
94
Without the use of a tracer, fertilizer use efficiency is often calculated via NUE indices
and the most appropriate calculation (Van Eerd 2007) was applied to the field plot data (total
above-ground plant N content in the fertilized subtracted by that of the zero fertilized treatments
and divided by N fertilizer applied). The above-ground broccoli biomass had a mean NUE of
26.8%, which was lower than the fertilizer recoveries based on the 15N tracer study (54.3%).
Previously with fertilizer applications of 500 to 625 kg N ha-1, apparent fertilizer N recoveries
(regression of plant N content at harvest by soil N fertilizer applied) ranged from 20 to 44% for
cole crops (Zebarth et al. 1991, 1995). However, a conclusion one draws on fertilizer use
efficiency depends on the index used (Van Eerd 2007). This 15N tracer study suggests that
broccoli plants may have taken up more fertilizer-derived N than previously thought using
common non-tracer indices (such as NUE or apparent N recoveries).
At harvest, the current 15N study found 89.4% of FN was recovered in plant biomass and
0-60 cm soil (not including the rhizosphere soil) (Table 4.2), which indicates that very little FN
was lost during the growing season. In the 0-60 cm soil, 35.1% of the FN was recovered at
harvest (Table 4.2), which is consistent the typical fertilizer recovery estimates of 30% in
temperate-zone soils by harvest (Kelley and Stevenson 1996). Up to 92.0 kg N ha-1 was found as
soil mineral TN at broccoli harvest in the 0-60 cm profile (Table 4.2). Similarly, it was suggested
that residual fertilizer or indigenous soil N was a significant source (> 100 kg ha-1 of soil mineral
N) for potential post-harvest N losses (Zebarth et al. 1995; Congreves et al. 2013a). However,
only 18.7 kg ha-1 of FN was inorganic at harvest (Table 4.2). Thus the majority (up to 73.3 kg ha1
) of soil mineral N was derived from indigenous mineralization rather than residual fertilizer-
derived N.
95
A considerable portion (84.0 and 94.9%) of soil total FN was organic in the top 60 cm
and rhizosphere, respectively (Table 4.2). Although it is difficult to precisely quantify the
rhizosphere soil N quantities, the method recommended by Gregory (2006) provided a
reasonable estimate. Either within the 0-60 cm soil profile or rhizosphere soil, this 15N study
suggests that RN at broccoli harvest was not readily susceptible to losses, because organic N
must be mineralized before subject to leaching or denitrification.
Broccoli roots had a C:N ratio of 26:1. Chaves et al. (2004) found that cole crop roots
resulted in net mineralization with C:N ratios of 37:1 or less, whereas a C:N ratio greater than
37:1 resulted in net immobilization. But, broccoli roots contained only 21.0 kg TN ha-1 (Table
4.2). Thus, the risk of post-harvest leaching or denitrification appeared greatest for the aboveground crop residue, which had a C:N ratio of 12:1 and if 37 to 60% TN mineralized (De Neve et
al. 1996; Congreves et al. 2013b) up to 172 kg TN ha-1 may readily leach or denitrify. Therefore,
N fate of above-ground crop residue-derived N after harvest warrants investigation to develop
better management strategies.
4.3.2
Fate of Above-ground Broccoli Crop Residue Derived Nitrogen
Two weeks after broccoli harvest in autumn, used cooking oil reduced 0-30 cm soil
mineral CRN by 19.4 kg ha-1 and increased microbial biomass CRN by 20.9 kg ha-1, compared to
the un-amended control (Table 4.3). Similarly, the CRN recovery was 7.62% less and 8.65%
greater in the mineral and microbial biomass pools, respectively, with vs. without used cooking
oil (Table 4.3). Therefore, it appears that CRN was rapidly immobilized during decomposition
which reduced the potential for N losses after both early and late broccoli harvest. Similarly,
used cooking oil reduced mineral TN, soil mineral N and NO3--N compared to the un-amended
96
control by 36.1, 22.9, and 22.2 kg ha-1, respectively (Table 4.3, 4.4). Previously, used cooking oil
reduced soil mineral N compared to the typical grower practice by 53 to 112 kg ha-1 or 30 to
50% and thereby reduced potential N losses after broccoli harvest from the top 30 cm soil layer
(Congreves et al. 2013a). Rashid and Voroney (2003) found that applications of oily food waste
in autumn reduced soil NO3--N by 47 to 56 kg N ha-1 in the top 60 cm. Therefore, immobilization
was greater in the previous studies (47 to 112 kg N ha-1) (Rashid and Voroney 2003; Congreves
et al. 2013a) than the presented results (19.4 to 36.1 kg N ha-1). In addition to the amount of oil
applied (i.e. 5 vs. 10 Mg ha-1 in the present study vs. that of Rashid and Voroney (2003)
respectively), the discrepancy could be a reflection of the environmental influence on
mineralization and immobilization rates, such as soil temperature and moisture. Nonetheless, the
present study suggested that 19.4 kg N ha-1 derived from crop residue may have been less
susceptible for leaching or gaseous losses throughout autumn from the top 30 cm soil.
97
Table 4.3 The fate of above-ground crop residue-derived N with vs. without used cooking oil two weeks after early and late broccoli
production (Sept or Oct 2011) and prior to soil freeze-up (Nov 2011) .
Crop Residue-Derived N
Crop Residue-Derived N
Total Stable Isotope N
(kg N ha-1)
(% recovery)
(kg N ha-1)
Soil N pool
Sample date
Broccoli production
Soil depth (cm)
Control
Oil
SE§
Control
Oil
SE§
Control
Oil
SE§
Total N
Sept/Oct 2011
Pooled
0-60
163
126
22.3
64.1
50.4
6.65
13116
12242
770.34
Nov 2011
Pooled
0-60
154
195
29.5
59.7
79.6
12.1
12716
12025
624.50
Sept/Oct 2011
Pooled
0-30
26.4
7.02***
3.31
10.6
2.98***
0.993
91.4
55.3*
8.3
Pooled
30-60
5.24
2.24
0.937
2.02
0.955
0.338
26.6
28.5
1.85
Early
0-60
37.8
18.1*
3.74
18.0
8.08‡
2.72
-
-
-
Late
0-60
26.1
36.4
7.6
9.20
13.7
3.08
-
-
-
Pooled
0-60
-
-
-
-
-
-
122
114
8.66
Pooled
0-30
3.65
24.5*
7.11
1.19
9.84*
2.92
Early
0-30
-
-
-
-
-
-
<0
171**
88.8
Late
0-30
-
-
-
-
-
-
210
210
80.2
Nov 2011
Pooled
0-30
36.7
12.4
20.2
12.0
4.33
7.85
210
31.9
191
Sept/Oct 2011
Pooled
0-60
131
117
21.3
51.5
46.4
6.84
12998
12158
772.89
Nov 2011
Pooled
0-60
122
168
30.9
46.2
68.7
12.4
12594
11911
628.10
Mineral N
Nov 2011
Microbial N
Organic N
Sept/Oct 2011
***, **, *, and ‡ symbols show statistical significance at P<0.001, 0.01, 0.05, and 0.1, respectively, with mean comparison based on an LSD
contrast to the control.
§
Standard error
98
Table 4.4 The 0-60 cm nitrate-nitrogen (NO3--N) and soil mineral N (SMN) from broccoli production (early and late pooled) in field plots
during autumn (Sept/Oct 2011), at spring wheat planting (Apr 2012) †, and at spring wheat harvest (July 2012)††.
Treatments
N fertilizer applied to broccoli
Broccoli residue incorporated
N fertilizer applied to spring wheat
Soil N pool
NO3--N
Soil mineral N
Broccoli crop
residue control
N fertilizer to
spring wheat
No N fertilizer to
broccoli
Broccoli crop
residue removed
Used cooking oil
amendment
yes
yes
no
yes
yes
yes
yes
yes
no
yes
no
yes
no
no
no
Sample date
Standard error
Sept/Oct 2011
43.6
-
9.69 ***
30.2 ns
21.4**
6.49
Apr 2012
76.1
-
63.6 ns
70.3 ns
56.3 ns
16.8
July 2012
2.68 -/**
25.1**/-
2.19 ns/**
1.19 ns/**
2.27 ns/**
4.80
Sept/Oct 2011
47.7
-
13.6***
34.2 ns
24.8**
6.67
Apr 2012
78.5
-
68.0 ns
72.0 ns
59.4 ns
16.9
July 2012
6.56 -/**
28.7**/-
6.15 ns/*
4.89 ns/**
5.72 ns/**
4.97
†Multiple mean comparisons based on a Dunnett-Hsu contrast against “broccoli crop residue control”.
†† Multiple mean comparisons based on a Dunnett-Hsu contrast against the “broccoli crop residue control” (symbol before slash) and “broccoli
crop residue with N fertilizer to wheat” (symbol after slash)
***, **, *, symbols show statistical significance at 0.001, 0.01, 0.05, and ns shows not significant.
99
By November in the 0-60 cm depth after early broccoli harvest there was 19.7 kg ha-1 less
mineral CRN with vs. without used cooking oil, and a similar trend was observed for CRN
recovery (Table 4.3). About 18 kg ha-1 more microbial CRN was correspondingly found with vs.
without used cooking oil, indicating CRN immobilization, although the difference was not
significant. It is possible that less soil mineral CRN was susceptible to over-winter N losses due
to the used cooking oil after early broccoli harvest. However, after the late-harvest broccoli
production, there was a lack of treatment difference in mineral CRN, indicating that
environmental conditions (i.e. lower soil temperatures) later in autumn may not have favored
decomposition of used cooking oil and broccoli crop residue (Table 4.3), in contrast to
Congreves et al. (2013a). Regardless, in soil total CRN or TN pools, no differences were
observed between used cooking oil and the control, indicating no differences in quantity of N
losses such as denitrification or NO3--N leaching by November.
At spring wheat planting, no differences in soil total or mineral CRN were found with or
without used cooking oil (Table 4.5). However, by spring wheat harvest in July 2012 there was
indication (P<0.1) that used cooking oil had 43.5 and 42.9 kg ha-1 greater soil total and organic
CRN, respectively, and recovered 19.0% more total CRN in the top 30 cm compared to the unamended control (Table 4.5). In the 0-60 cm depth by Jul 2012, used cooking oil had 0.75 kg ha-1
more mineral CRN (P<0.05) with 0.31% greater recovery of mineral CRN (P<0.1) compared to
without oil (Table 4.5). Relative to the control and over time, used cooking oil increased 0-60 cm
soil total, mineral, and organic CRN according to a positive first order exponential model (Figure
4.1). At spring wheat harvest, it was estimated that soil total, mineral, and organic CRN
increased by 190, 190, and 209%, respectively, due to used cooking oil (Figure 4.1). Therefore,
used cooking oil appeared to reduce soil CRN losses over the year.
100
However, no differences were observed with vs. without used cooking oil in the total or
mineral TN pool by July 2012 (Table 4.5), similar to the soil mineral N and NO3--N field plot
results (Table 4.4) and previous research by Congreves et al. (2013a). Thus, without the use of
the 15N tracer it would not have been possible to identify that more crop residue-derived N (soil
total, mineral, or organic) remained at spring wheat harvest due to the used cooking oil
amendment. Hence, using stable isotope measurements for source-sink processes can better
address complex problems like reducing N losses (Peterson and Fry 1987).
101
Table 4.5 The fate of above-ground crop residue-derived N with vs. without used cooking oil after early and late broccoli production in
soil at spring wheat planting (Apr 2012) and harvest (July 2012).
Crop Residue-Derived N
Crop Residue-Derived N
Total Stable Isotope N
(kg N ha-1)
(% recovery)
(kg N ha-1)
Soil N pool
Sample date
Broccoli production
Soil depth (cm)
Control
Oil
SE§
Control
Oil
SE§
Control
Oil
SE§
Total N
Apr 2012
Pooled
0-60
111
151
26.3
41.7
58.8
9.94
12813
12010
765.77
Jul 2012
Pooled
0-30
40.4
83.9‡
14.7
16.7
35.7‡
6.54
-
-
-
Pooled
30-60
21.9
18.8
6.81
8.90
7.91
2.69
-
-
-
Pooled
0-60
-
-
-
-
-
-
11898
12255
496.97
Apr 2012
Pooled
0-60
18.6
14.5
3.40
7.02
5.64
1.14
93.9
84.2
6.33
Jul 2012
Pooled
0-60
1.02
1.77*
0.276
0.424
0.733‡
0.121
47.6
54.0
3.53
Apr 2012
Pooled
0-60
92.0
137
26.6
34.6
53.1
10.0
12719
11956
769.93
Jul 2012
Pooled
0-30
39.6
82.5‡
14.5
-
-
-
-
-
-
Pooled
30-60
21.7
18.5
6.81
-
-
-
-
-
-
Pooled
0-60
-
-
-
25.2
42.9
8.04
11850
12201
498.22
Mineral N
Organic N
***, **, *, and ‡ symbols show statistical significance at P<0.001, 0.01, 0.05, and 0.1, respectively, with mean comparison based on an LSD
contrast to the control.
§
Standard error
102
A) y = 190(1-exp-0.51(time)), R2=0.34, P<0.0001
B) y = 190(1-exp-0.39(time)), R2=0.40, P<0.0001
C) y = 209(1-exp-0.62(time)), R2=0.35, P=0.0006.
Figure 4.1 The effect of used cooking oil on 0-60 cm soil A) total, B) mineral, and C) organic Nderived from crop residue (kg ha-1) expressed as a percentage (%) of the crop residue-derived N (kg
ha-1) in control over time (months). Bars represent the experimental means with standard error and
103
the solid line represents the prediction according to the first order exponential model y =A(1-expk(time)
), (y=percent of control, A=coefficient indicates potential N, k=rate constant, t=time).
At spring wheat harvest, grain yields ranged from 2.60 to 4.07 Mg ha-1 (Table 4.6, 4.7),
comparable to the average Ontario spring wheat yield of 3.7 Mg ha-1 in 2012 (OMAFRA 2012)
and therefore representative of typical production. With vs. without used cooking oil, no
differences were observed in CRN, TN, yield or biomass for grain, straw, or total plant tissue at
spring wheat harvest (Table 4.6). Thus, the greater soil CRN with used cooking oil in July 2012
(Table 4.5) did not translate into differences between treatments for spring wheat plant N
dynamics or in grain yields (Table 4.6). Nonetheless, used cooking oil at broccoli harvest did not
negatively affect spring wheat production (Table 4.6, 4.7), as was also found by Congreves et al.
(2013a). However, the highest spring wheat yields were obtained by applying pre-plant N
fertilizer to spring wheat (Table 4.7). Although there was no economic or agronomic benefit of
applying used cooking oil to a soil with a non-limiting N status (Congreves et al. 2013a), one
may expect less of an economic disadvantage with used cooking oil amendments on N limited
soils and if re-mineralized N contributed to spring wheat crop yield.
104
Table 4.6 The fate of above-ground crop residue-derived N with vs. without used cooking oil after early and late broccoli production in
spring wheat plant tissue at harvest (Jul 2012)†.
Yield or Biomass
Crop Residue-Derived N
Crop Residue-Derived N
Total Stable Isotope N
(Mg ha-1)
(kg N ha-1)
(% recovery)
(kg N ha-1)
Plant tissue
Broccoli production
Control
Oil
SE§
Control
Oil
SE§
Control
Oil
SE§
Control
Oil
SE§
Grain yield
Pooled
2.86
3.15
0.208
14.9
15.6
1.60
5.95
6.20
2.70
89.1
97.1
6.71
Straw biomass
Pooled
5.15
4.42
0.502
6.61
5.44
0.938
2.70
2.18
0.376
43.8
35.3
4.68
Total plant biomass
Pooled
8.01
7.57
0.668
21.5
21.1
2.34
8.66
8.38
0.869
133
132
10.5
† Mean comparison based on an LSD contrast to the control, no significant differences were found at P<0.1.
§
Standard error
105
Table 4.7 2012 spring wheat harvest parameters following after early and late broccoli production and field plot treatments†.
Treatments
N fertilizer applied to broccoli
Broccoli residue incorporated
N fertilizer applied to spring wheat
Broccoli crop
residue control
N fertilizer to
spring wheat
No N fertilizer to
broccoli
Broccoli crop
residue removed
Used cooking oil
amendment
yes
yes
no
yes
yes
yes
yes
yes
no
yes
no
yes
no
no
no
Harvest parameter
Broccoli production
Standard error
Grain N (kg ha-1)
Pooled
103
126
102
108
74.6
12.1
Straw N (kg ha-1)
Early
60.9
66.9
72.7
38.8
33.7 ns/*
8.31
Late
57.2
61.9
54.5
80.4
45.1
9.78
Plant N (kg ha-1)
Pooled
162
191
165
176
114 ns/**
15.9
Grain Yield (Mg ha-1)
Pooled
3.16
4.07
3.74
3.65
2.60 ns/**
0.292
Straw Biomass (Mg ha-1)
Pooled
4.94
5.81
5.48
5.60
4.41 ns/*
0.387
Plant Biomass (Mg ha-1)
Early
7.86
9.45
10.2 */ns
7.99
6.92 ns/*
0.629
Late
8.33
10.3
8.24
11.2 */ns
7.10 ns/**
0.697
† Multiple mean comparisons based on a Dunnett-Hsu contrast against the “broccoli crop residue control” (symbol before slash) and “broccoli
crop residue with N fertilizer to wheat” (symbol after slash).
***, **, *, symbols show statistical significance at P<0.001, 0.01, 0.05, and ns shows not significant.
106
Some re-mineralization of CRN may have occurred during the spring wheat growing
season following broccoli harvest and applying used cooking oil (Table 4.5, Figure 4.1). Thus,
used cooking oil did not cause prolonged net N immobilization which could have decreased N
uptake by the subsequent crop. Likely, used cooking oil was readily decomposable, promoted
rapid microbial growth and N immobilization during autumn, which was followed by a cessation
of microbial activity and re-mineralization during subsequent growing season. Likewise, a
previous incubation study found that used cooking oil promoted greater microbial activity than
other C amendments of wheat straw or yard waste, and showed a pattern of N re-mineralization
(Congreves et al. 2013b). Others have related N re-mineralization to high microbial growth rates
and NH4+ concentrations (Bengtson and Bengtsson 2005) and re-mineralization was observed
during the subsequent growing season after fall oil amendments (Rashid and Voroney 2003).
Immobilized CRN (which could have otherwise been lost) in 2011 due to used cooking oil might
have re-mineralized during the spring wheat growing season in 2012. Future research should
address the risk of crop residue-derived N losses for a longer period after spring wheat harvest in
July.
Pool substitution between 14N and 15N may cause 15N retention in the stable organic soil
fraction (Jenkinson et al. 1985; Rao et al, 1991; Timmons and Cruse 1991; Kuzyakov et al. 2000;
Gabriel and Quemada 2011). However, the observed increase in soil organic CRN over time due
to used cooking oil (Figure 4.1) was likely not a pool substitution artifact because it is well
known that easily decomposable C material (i.e. used cooking oil, glucose, or cellulose) have
increased microbial activity and contributed to the formation of soil organic matter (Kuzyakov et
al. 2000). A considerable quantity of 15N enriched fertilizer may become integrated into stable
organic pools; not readily available for microbial transformation, plant uptake, leaching or
107
gaseous losses (Timmons and Cruse 1991; Janzen et al. 1992; Kaye et al. 2002). Management
practices which have increased microbial activity have also increased the potential to retain soil
N (Bosshard et al. 2008). Previously immobilized soil N was incorporated into insoluble
components of microbial tissues such as fungal melanins (He et al. 1988), and stable N organic
compounds have been promoted by soil aggregation (Hassink et al. 1993). Others have found
rice straw amendments increased the amount of fertilizer-15N remaining in recalcitrant soil
organic compounds after 160 d (Devêvre and Horwáth 2001). Because used cooking oil
amendments increased soil microbial activity (Plante and Voroney 1998; Congreves et al.
2013b), aggregate stability (Plante and Voroney 1998), and soil organic CRN over time (Fig.
4.1), CRN was likely integrated into recalcitrant organic compounds thus not readily available
for leaching or gaseous losses. Therefore, growers should amend soil with used cooking oil after
cole crop harvest to reduce N losses and also to increase the soil organic N fraction. Future
research should explore the effect of used cooking oil on labile and stable soil organic N
compounds.
4.3.3
Fate of Residual Fertilizer or Broccoli Root Derived Nitrogen
After broccoli harvest during autumn 2011, quantities of total, mineral, or organic RN
were similar with vs. without used cooking oil, in contrast to the treatment effect with TN (Table
4.8) or CRN (Table 4.3) and previous research (Congreves et al. 2013a; 2013b). The chemical
form and quantity of RN likely contributed to the lack of treatment effect. Soil organic RN
represented over 89% of total RN two weeks after harvest (Table 4.8). It is possible that the
majority of RN was incorporated into stable organic N compounds and largely resistant to
manipulation of N mineralization-immobilization dynamics by amending soil with used cooking
108
oil. Also, there was approximately 56.9% less total RN than CRN during autumn, so perhaps
there was too little RN to detect differences based on innate soil variability. It has generally been
assumed that N dynamics and losses from cole crop roots or residual fertilizer N were similar to
those from above-ground crop residue (Zebarth et al. 1995); however the present study suggests
otherwise.
Soil N mineralization or immobilization dynamics of vegetable root residues is largely
unknown. A study of cole crop (red cabbage, white cabbage, savoy cabbage, and Brussels
sprouts) root decomposition found net N mineralization for all fine cabbage root residues, but N
immobilization for Brussels sprout and larger cabbage root residues (> 1 cm diameter) (Chaves
et al. 2004). After removing above-ground broccoli crop residue, previous researchers
hypothesized that residual soil fertilizer N or root biomass was a contributing factor to high
autumn soil mineral N concentrations (89 to 227 kg N ha-1) (Congreves et al. 2013a).
Interestingly, when all above-ground broccoli biomass was removed from the field, similar soil
mineral N and NO3--N quantities were observed during autumn 2011 as when the crop residue
was incorporated into the field (Table 4.4), indicating a source of mineral N other than the
above-ground crop residue. Estimated by the current 15N study, up to 84.1 kg N ha-1 was
mineralized from indigenous soil N by November (0-60 cm TN subtracted by the sum of soil
mineral RN and CRN). Thus, indigenous soil mineralization during autumn produced a greater
source of N available for potential losses than did the residual fertilizer.
109
Table 4.8 The fate of residual fertilizer or broccoli root-derived N with vs. without used cooking oil
two weeks after early and late broccoli production (Sept or Oct 2011), prior to soil freeze-up (Nov
2011), at spring wheat planting (Apr 2012) and spring wheat harvest (Jul 2012).
Soil N
Broccoli
Residual Fertilizer or Root-
Total Stable Isotope N
Derived N (kg N ha-1)
(kg N ha-1)
Soil depth
Control
Oil
SE§
Control
Oil
SE§
pool
Sample date
production
(cm)
Total N
Sept/Oct 2011
Pooled
0-60
47.4
55.0
5.83
11628
11779
457.49
Nov 2011
Pooled
0-60
79.2
94.6
13.3
12770
13256
1247.68
Apr 2012
Pooled
0-60
53.1
53.4
7.12
11842
12689
785.52
Jul 2012
Pooled
0-60
48.0
46.5
7.76
13083
11572
732.42
Sept/Oct 2011
Pooled
0-60
9.40
11.0
3.47
116
78.6*
12.2
Nov 2011
Pooled
0-60
32.1
36.5
6.02
154
151
17.0
Apr 2012
Early
0-60
2.97
5.09
0.441
-
-
-
Late
0-60
5.44
2.86
0.848
-
-
-
Pooled
0-60
-
-
-
106
106
6.07
Pooled
0-60
0.756
0.683
0.0972
69.0
68.8
4.19
Sept/Oct 2011
Pooled
0-60
42.4
50.5
13.4
10929
11656
550.29
Nov 2011
Pooled
0-60
46.4
56.5
12.5
13399
12482
549.33
Apr 2012
Pooled
0-60
50.7
46.1
4.87
12310
12000
441.48
Jul 2012
Pooled
0-60
43.7
53.9
4.69
12199
12330
586.75
Mineral N
Jul 2012
Organic N
* symbol shows statistical significance at P<0.05 with mean comparison based on an LSD test and
contrast to the control.
§
Standard error.
At spring wheat planting or harvest in 2012, quantities of soil total, mineral, or organic
RN or TN were similar with and without used cooking oil (Table 4.8). Regardless of treatment,
over 91% of total RN was organic (Table 4.8), and therefore not likely readily available to spring
110
wheat N uptake (Table 4.9). Previous 15N tracer research suggested that a large fraction of
legume root-derived N remained insoluble after lupin harvest, possibly in fine root biomass
within the soil (Russell et al., 1996). Although some (20%) lupin root-derived N was mineralized
at subsequent wheat seeding, up to 55% remained in a form which was not readily plantavailable for the duration of the subsequent wheat season and was considered to be an important
contribution to structural and nutritional long-term sustainability of soils (Russell et al. 1996).
Other 15N research found that residual fertilizer and cover crop roots contributed significantly
less N than cover crop shoots to subsequent cereal crops (Glasener et al. 2002), suggesting that
root N was not a significant source for subsequent plant uptake or losses. Roots have contributed
more to the stable soil C than above-ground residue-derived C (Kätterer et al. 2011). Thus,
residual fertilizer or cole crop root biomass derived-N may have been largely resistant to postharvest losses. Cole crop growers and researchers should therefore focus on above-ground crop
residue N management.
111
Table 4.9 The fate of residual fertilizer or broccoli root-derived N with vs. without used cooking oil
after early and late broccoli production in spring wheat plant tissue at harvest (Jul 2012)†.
Yield or Biomass
Residual Fertilizer or
Total Stable Isotope N
Root-Derived N
(kg N ha-1)
(Mg ha-1)
(kg N ha-1)
Plant tissue
Broccoli production
Control
Oil
SE§
Control
Oil
SE
Control
Oil
SE
Grain Yield
Pooled
3.06
3.16
0.121
6.59
7.17
0.840
90.0
92.2
4.55
Straw Biomass
Pooled
4.64
4.57
0.363
2.59
2.60
0.520
35.6
32.0
4.29
Total Plant Biomass
Pooled
-
-
-
9.18
9.77
1.27
-
-
-
Early
8.67
8.33
0.477
-
-
-
146
133
11.5
Late
6.73
7.12
0.744
-
-
-
105
116
11.2
† Mean comparison based on an LSD contrast to the control, no significant differences were found at
P<0.1.
§
Standard error.
4.4 CONCLUSIONS
By using a 15N tracer, this study has demonstrated greater N fertilizer uptake for broccoli
crops than previous thought using common non-tracer NUE indices. At broccoli harvest very
little fertilizer N was lost during the growing season, however over 400 kg N ha-1 of residual N
(leaf, stem, root N, and 0-60 cm soil mineral N) may be susceptible to losses after broccoli
harvest. Therefore it is crucial that better management strategies, such as amending soil with
used cooking oil, are practiced after cole crop production.
The present study was the first to partition above- and below-ground residual N at
broccoli harvests, and showed that used cooking oil reduced above-ground residue-N losses by
up to 209% over a year. Used cooking oil had no impact on subsequent spring wheat production
112
compared to the un-amended control, thus growers may reduce N losses without compromising
subsequent yield.
It is hypothesized that used cooking oil facilitated the incorporation of above-ground crop
residue-derived N into stable organic compounds, thereby reducing potential losses. Little is
known of the contribution of roots to post-harvest N dynamics, and it has been assumed that N
losses from roots or residual fertilizer N were similar to that of above-ground crop residue.
However, this study has demonstrated the difference between below-ground and above-ground
residual N dynamics. Notably, RN quantities were lower than CRN, RN was mostly organic and
largely resistant to post-harvest losses. Therefore, research should focus on reducing aboveground crop residue N losses.
It is recommended that growers apply used cooking oil at cole crop harvest to minimize
potential above-ground crop residue N losses and to increase the soil organic N fraction. Future
research should explore the effect of used cooking oil on stable and labile organic N compounds.
113
5
CONCLUSIONS AND RECOMMENDATIONS
Most soil nutrient BMPs focus on minimizing N losses during fertilizer application or the
growing season, but for cole crops the greatest risk of N losses is likely after harvest due to the
high content of readily mineralizable N in crop residues. Over 400 kg N ha-1 of residual N (leaf,
stem, root N, and 0-60 cm SMN) may be susceptible to losses after broccoli crop harvest.
Therefore this research was focused on post-harvest N dynamics to develop a novel BMP to
reduce N losses after broccoli crop harvest.
Estimates for crop NUE help inform N management practices. With the use of a 15N
tracer, this study has demonstrated greater N fertilizer uptake for broccoli crops than previous
NUE estimates (Chapter 4). Therefore this research suggests that broccoli plants may take up
more fertilizer-derived N than previously thought using common non-tracer indices. In general,
little is known of fertilizer recovery in roots. However, based on this 15N study, a shoot to root
FN ratio of 19:1 may be used by future researchers to estimate total plant uptake and fertilizer N
recovery in broccoli production (Chapter 4). Therefore, the presented research contributes to the
knowledge of fertilizer N use in horticulture.
This research evaluated the effects of soil organic C amendments (wheat straw, yard
waste, and used cooking oil) on N dynamics after broccoli production. The synchrony of cole
crop residue and organic C amendment decomposition is crucial for the immobilization of N
derived from crop residue, and therefore necessary for the development an effective BMP to
reduce N losses. While previous research has not matched OCA decomposition rates with that of
vegetable crop residues, this dissertation has included the first study to demonstrate that used
cooking oil had a synchronous decomposition rate with broccoli crop residue, a synergistic effect
114
on N immobilization and microbial activity when incorporated with broccoli crop residue rather
than fertilizer, and had the greatest levels of net N immobilization (86 mg kg-1 soil) (Chapter 2).
Therefore, this research has contributed to our understanding of organic matter decomposition. In
disciplines not limited to horticulture, future researchers should use the presented research to
inform the selection of appropriate amendments for reducing potential N losses.
The most sustainable BMPs should reduce nutrient losses in situ without producing yield
deficits. Although wheat straw and yard waste showed net N immobilization under controlled
conditions after the incorporation with broccoli crop residue (Chapter 2), there was inconsistency
in reducing potential N losses after broccoli harvest in the field (Chapter 3). Furthermore, spring
wheat yield was reduced following wheat straw amendments. Therefore, it is not recommend
that growers apply wheat straw or yard waste as a BMP after broccoli harvest (Chapter 3).
Notably, based on the incubation (Chapter 2), field (Chapter 3), and 15N tracer (Chapter
4) research, it is recommended that growers apply used cooking oil as a BMP for reducing N
losses after broccoli crop harvest. Used cooking oil reduced potential crop residue N losses via
immobilization (Chapter 2, 3, 4) and also increased the soil organic N fraction (Chapter 4). With
used cooking oil, potential N losses may be reduced by up to 112 kg N ha-1 (Chapter 3) without
affecting spring wheat yield or N content, compared to no oil amendment (Chapter 3, 4). After ≈
one year, used cooking oil reduced above-ground crop residue derived-N losses by 190%.
Therefore, this dissertation has contributed to N management research, and presents a novel
strategy for reducing N losses after broccoli production.
Although there was no economic or agronomic benefit of applying used cooking oil to a
soil with a non-limiting N status (Chapter 3), one may expect less of an economic disadvantage
115
with used cooking oil amendments on N limited soils and if re-mineralized N contributed to
subsequent crop uptake. Although the presented studies were conducted on soil types typically
present in southern Ontario (i.e. clay loam Gleysolic soils), sandier soils may leach N more
readily than clay loam soils after broccoli production. Thus, future research should explore the
agronomic and economic effects of used cooking oil amendments on N limited soil or sandy
soils. Long-term yearly applications of used cooking oil may impact agronomic or economic
benefits, perhaps by enhancing soil N fertility and organic matter content and therefore requires
future study.
Future researchers should investigate the effect of used cooking oil on stable and labile
organic N (Chapter 4). Although the present research did not differentiate stable and labile
organic N, it is hypothesized that used cooking oil facilitated the incorporation of residue-derived
N into stable organic forms, thereby reducing leaching or gaseous N losses (Chapter 4). It is
possible that immobilized N, due to the used cooking oil amendment, re-mineralizes after spring
wheat harvest and future researchers are encouraged to address the risk of broccoli crop residuederived N losses for a longer period after spring wheat harvest.
Generally, researchers have largely ignored the contribution of roots to post-harvest N
dynamics and it has been assumed that N losses from roots or residual fertilizer N were similar to
that of above-ground crop residue. However, this presented 15N research has suggested otherwise
by demonstrating the difference between below-ground and above-ground residual N dynamics
(Chapter 4). Therefore, this research contributes to the knowledge and understanding of belowground crop residue and post-harvest N losses. Below-ground residue N was mainly in the soil
organic N fraction, remained relatively stable over a year, and had lower quantities than above-
116
ground residual N (Chapter 4). Thus, below-ground residual N was largely resistant to postharvest losses. Researchers and cole crop growers should therefore focus on above-ground crop
residue N management (Chapter 4).
The practice of amending soil with used cooking oil to reduce potential N losses after
broccoli production may be relevant for other cole crops. Cabbage, cauliflower, or Brussels
sprouts belong to the same species at broccoli (Brassica oleracea L.) and may leave comparable
quantities of crop residue and mineralizable N in the field after harvest as broccoli. Therefore, it
is predicted that used cooking oil amendments reduce potential N losses after cabbage,
cauliflower, and Brussels sprout harvest.
As society or policy demands more environmental accountability from farmers, growers
may be motivated by social, economic, or regulatory factors to adopt management practices
which minimize nutrient losses (Beegle et al. 2000). The effectiveness of used cooking oil
amendments as a novel BMP provides growers, policy, and future research with a useful method
for reducing N losses and thereby environmental N contamination.
117
6
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7
APPENDICES
APPENDIX A ALTERNATIVE STRATEGIES TO REDUCE NITROGEN LOSSES
AFTER COLE CROP HARVEST
Within the trial described in Chapter 3, other treatments were included and focused on
managing crop residue rather than applying high carbon organic wastes and included crop
residue removal, modification of crop residue structure, crop residue left on soil surface, crop left
alive (Table A.2). The full report is available on the OMAFRA website:
http://www.omafra.gov.on.ca/english/research/new_directions/projects/2009/sr9226.htm.
Table A.1 Treatment list of strategies to reduce nitrogen loss.
Treatment
Description
Incorporation of broccoli crop residue
The positive control treatment. Broccoli crop
residue (leaves and stems) were mulched and
disked into soil after harvest.
Incorporation of residue + pre-plant N fertilizer The grower typical treatment. Broccoli crop
to following spring wheat crop
residue (leaves and stems) were mulched and
disked into soil after harvest. Fertilizer was
applied to spring wheat before planting.
Physical removal of residue
The negative control treatment. All aboveground crop residue was removed from soil
surface by hand after broccoli harvest.
Leaving crop residue on soil surface.
Crop residue was mulched and left on soil
surface after broccoli harvest.
136
Alter cole crop plant structure/architecture in-
The four to five leaves near the lower stem
season to minimize crop residue at harvest
were removed from crop twice during the
growing season.
Leave the crop standing and alive in the field
The crop residue was not mulched and was
allowed to continue growth after broccoli
harvest.
Organic carbon amendments of wheat straw,
Described in Chapter 3.
yard waste, and used cooking oil.
Relative to the incorporation of crop residue control, the strategies which reduced autumn
soil nitrate (NO3--N) levels were: removing crop residue, leaving crop residue on surface, or
leaving the crop alive in two out of four trials, or: by the modified crop residue structure in one
out of four trials (Figures A.1, A.2, A.3, A.4). Subsequent spring wheat yield and grain N content
were reduced by treatments of crop residue removal and crop left standing (Figure A.4 and
Figure A.5). Thus, there may not be a large advantage to growers to implement the alternative
strategies.
137
400
Soil Nitrate kg N ha-1
350
Crop residue
incorporation
300
Crop residue removal
250
200
Crop residue modified
structure
150
Crop residue left on
surface
Crop residue left standing
100
*
*
*
*
50
0
Harvest
Aug.21
Sept.18
Oct.22
Sample Day
Figure A.1 In 2009, the effect of alternative post-harvest strategies on soil nitrate concentrations
after early harvested broccoli (4 Aug). Based on the last sample day, *asterik denotes a statistical
difference between treatment and incorporated crop residue (control).
400
Soil Nitrate kg N ha-1
350
Crop residue
incorporation
300
Crop residue removal
250
200
Crop residue modified
structure
150
Crop residue left on
surface
100
Crop residue left standing
50
*
0
Harvest Aug.20 Sept.1 Sept.22 Oct.25
Sample Day
Figure A.2 In 2010, the effect of alternative post-harvest strategies on soil nitrate concentrations
after early harvested broccoli (3 Aug). Based on the last sample day, *asterik denotes a statistical
difference between treatment and incorporated crop residue (control).
138
200
Soil Nitrate kg N ha-1
180
160
140
Crop residue
incorporation
120
Crop residue removal
100
Crop residue modified
structure
80
60
40
Crop residue left on
surface
20
Oat cover crop
0
Harvest
Oct.26
Sample Day
Figure A.3 In 2009, the effect of alternative post-harvest strategies on soil nitrate concentrations
after late harvested broccoli (31 Aug). Based on the last sample day, all treatments were not
different from the incorporated crop residue (control).
200
Soil Nitrate kg N ha-1
180
160
140
Crop residue
incorporation
120
*
*
100
80
Crop residue removal
Crop residue modified
structure
60
40
Crop residue left on
surface
20
0
Harvest
Oct.6
Oct.25
Sample Day
Figure A.4 In 2010, the effect of alternative post-harvest strategies on soil nitrate concentrations
after late harvested broccoli (22 Sept). Based on the last sample day, *asterik denotes a statistical
difference between treatment and incorporated crop residue (control).
139
3
Crop residue incorporated
2.5
Yield (Mg ha-1)
*
Crop residue with full
spring N fertilizer
Crop residue removal
*
2
1.5
Crop residue modified
structure
Crop residue left on
surface
Crop left standing
1
0.5
0
1
Figure A.5 Effect of alternative post- broccoli harvest strategies on mean spring wheat grain yield
in 2010 and 2011. * Asterik denotes a statistical difference between treatment and incorporated
crop residue (control).
90
Crop residue incorporated
Grain Nitrogen (kg N ha-1)
80
70
*
Crop residue with full
spring N fertilizer
Crop residue removal
*
60
50
Crop residue modified
structure
Crop residue left on
surface
Crop left standing
40
30
20
10
0
1
Figure A.6 Effect of alternative post- broccoli harvest strategies on mean spring wheat grain N
content in 2010 and 2011. * Asterik denotes a statistical difference between treatment(*) and
incorporated crop residue (control).
140
APPENDIX B STATISTICAL TABLES FOR DATA IN CHAPTER 2
Table B.1 Analysis of variance using SAS Proc Mixed for the fixed effects of cumulative net N and
C mineralization shown in sections 2.4.1. and 2.4.2.
Cumulative net N mineralization
Fixed Effects
Pr>F
Treatment
<0.0001
Day
<0.0001
Treatment x Day
<0.0001
Day 1
<0.0001
Day 3
<0.0001
Day 7
<0.0001
Day 14
<0.0001
Day 21
<0.0001
Day 28
<0.0001
Day 42
<0.0001
Day 56
<0.0001
Cumulative net C mineralization
Fixed Effects
Pr>F
Treatment
<0.0001
Day
<0.0001
Treatment x Day
<0.0001
Day 2
<0.0001
Day 4
<0.0001
141
Day 7
<0.0001
Day 10
<0.0001
Day 13
<0.0001
Day 17
<0.0001
Day 22
<0.0001
Day 35
<0.0001
Day 42
<0.0001
Day 56
<0.0001
142
Table B.2 Least significance difference test using SAS Proc Mixed of treatment x day effect sliced
by sample day for cumulative net N mineralization (mg N kg-1), shown in Figure 2.1. Statistical
differences among rows indicated by different letters and case (P<0.05).
Amendment
Sample Day Estimate
Letter
(mg N kg-1)
Broccoli residue
Broccoli residue + wheat straw
Broccoli residue + yard waste
Standard
Error
1
32.9
EF
4.57
3
30.3
FG
4.57
7
62.2
BC
4.57
14
68.8
ABC
4.57
21
65.2
ABC
4.57
28
63.2
BC
4.57
42
76.8
A
4.57
56
67.0
ABC
4.57
1
-5.9
JKLMNO
4.57
3
9.6
HI
4.57
7
-15.2
NOPQRS
4.57
14
-26.4
STU
4.57
21
-33.0
U
4.57
28
-24.8
QRSTU
4.57
42
-29.7
U
4.57
56
-16.9
OPQRST
4.57
1
3.7
IJ
4.57
3
-2.3
IJKLM
4.57
143
Broccoli residue + used cooking oil
N fertilizer + wheat straw
N fertilizer + yard waste
7
-0.3
IJKL
4.57
14
-4.7
JKLMNO
4.57
21
-10.4
KLMNO
4.57
28
-9.3
KLMNO
4.57
42
-13.1
MNOPQR 4.57
56
-12.3
LMNOPQ 4.57
1
-10.4
KLMNO
4.57
3
-58.7
VW
4.57
7
-90.8
ab
4.57
14
-110.3
d
4.57
21
-112.8
d
4.57
28
-104.6
Cd
4.57
42
-92.3
bc
4.57
56
-86.0
Zab
4.57
1
-2.7
IJKLMN
4.57
3
-8.7
JKLMNO
4.57
7
-9.0
JKLMNO
4.57
14
-30.8
U
4.57
21
-24.1
PQRSTU
4.57
28
-25.3
RSTU
4.57
42
-31.7
U
4.57
56
-28.3
TU
4.57
1
0.9
IJK
4.57
144
N fertilizer + used cooking oil
Broccoli residue + N fertilizer
3
-3.3
JKLMN
4.57
7
-1.2
IJKLM
4.57
14
-9.1
KLMNO
4.57
21
-6.4
JKLMNO
4.57
28
-11.8
KLMNOP 4.57
42
-12.1
LMNOPQ 4.57
56
-9.9
KLMNO
4.57
1
0.2
IJKL
4.57
3
-54.7
V
4.57
7
-63.5
VWX
4.57
14
-76.2
XYZ
4.57
21
-69.7
WXY
4.57
28
-74.3
XYZ
4.57
42
-79.5
YZa
4.57
56
-66.2
VWX
4.57
1
28.8
FG
4.57
3
20.0
GH
4.57
7
44.7
DE
4.57
14
61.4
BC
4.57
21
56.9
CD
4.57
28
72.1
AB
4.57
42
73.1
AB
4.57
56
73.5
AB
4.57
145
Table B.3 Least significance difference test using SAS Proc Mixed of treatment x day effect sliced
by sample day for cumulative net C mineralization (mg N kg-1), shown in Figure 2.2. Statistical
differences among rows indicated by different letters and case (P<0.05).
Amendment
Broccoli residue
Broccoli residue + wheat straw
Sample Estimate
Letter
Standard Error
Day
(mg C kg-1)
2
9.8
XYZabcd
5.52
4
29.8
ST
5.52
7
45.9
OPQ
5.52
10
53.7
MNO
5.52
13
59.0
LMNO
5.52
17
61.7
LMN
5.52
22
63.6
LM
5.52
28
64.9
LM
5.52
35
65.7
KLM
5.52
42
65.7
KLM
5.52
56
66.2
KLM
5.52
2
-1.6
d
5.52
4
-3.1
d
6.30
7
5.2
YZabcd
6.30
10
5.4
Zabcd
5.52
13
8.8
XYZabcd
5.52
17
14.0
VWXYZabc
5.52
22
19.8
STUVWXY
5.52
146
Z
Broccoli residue + yard waste
28
25.1
STUVW
5.52
35
28.7
STU
5.52
42
30.1
RST
5.52
56
33.6
PQRS
5.52
2
1.1
cd
5.52
4
3.9
abcd
5.52
7
8.4
XYZabcd
5.52
10
10.6
WXYZabcd
5.52
13
10.8
WXYZabcd
5.52
17
12.6
VWXYZabc
5.52
d
22
14.1
VWXYZabc
5.52
28
16.1
TUVWXYZa
5.52
b
Broccoli residue + used cooking oil
35
17.0
TUVWXYZa
5.52
42
17.2
TUVWXYZa
5.52
56
20.7
STUVWXY
5.52
2
8.9
XYZabcd
5.52
4
32.5
QRS
5.52
7
58.9
LMNO
5.52
10
82.7
IJ
6.30
13
90.5
I
5.52
147
N fertilizer + wheat straw
17
108.8
FG
5.52
22
125.4
E
5.52
28
142.7
CD
5.52
35
156.2
BC
5.52
42
163.2
AB
5.52
56
176.4
A
5.52
2
0.0
cd
5.52
4
0.0
cd
5.52
7
6.3
YZabcd
5.52
10
11.3
WXYZabcd
5.52
13
14.1
UVWXYZab
5.52
c
17
17.1
TUVWXYZa
5.52
22
19.6
STUVWXY
5.52
Z
N fertilizer + yard waste
28
22.8
STUVWX
5.52
35
24.4
STUVW
5.52
42
24.4
STUVW
5.52
56
25.1
STUVW
5.52
2
0.0
cd
5.52
4
0.0
cd
5.52
7
0.6
cd
5.52
10
1.6
bcd
5.52
148
N fertilizer + used cooking oil
Broccoli residue + N fertilizer
13
3.6
abcd
5.52
17
3.6
abcd
5.52
22
3.7
abcd
5.52
28
3.7
abcd
5.52
35
3.7
abcd
5.52
42
3.7
abcd
5.52
56
4.0
abcd
5.52
2
0.2
cd
5.52
4
24.6
STUVW
5.52
7
47.2
NOP
5.52
10
67.8
JKLM
5.52
13
79.9
IJK
5.52
17
93.0
HI
5.52
22
105.5
GH
5.52
28
120.7
EF
5.52
35
133.5
DE
5.52
42
140.6
D
5.52
56
155.5
BC
5.52
2
7.3
YZabcd
5.54
4
27.0
STUV
5.54
7
44.7
OPQR
5.54
10
53.7
MNO
5.54
13
57.9
LMNO
5.54
149
17
63.2
LM
5.54
22
64.8
LM
5.54
28
66.5
KLM
5.54
35
67.1
JKLM
5.54
42
67.1
JKLM
5.54
56
68.5
JKL
5.54
150
10
5
0
1
8 15 22 29 36 43 50
Time (d)
Broccoli residue
Broccoli residue + wheat straw
Broccoli residue + yard waste
Broccoli residue + used cooking oil
Net C Mineralization Rate (mg C kg-1 soil d-1)
Net C Mineralization Rate (mg C kg-1 soil d-1)
15
15
10
5
0
1 8 15 22 29 36 43 50
Time (d)
Broccoli residue + ammonium nitrate
Ammonium nitrate + wheat straw
Ammonium nitrate + yard waste
Ammonium nitrate + used cooking oil
Figure B.1 Net C mineralization rate (mg C kg-1 dry soil) as affected by organic carbon amendment with broccoli residue- derived N (left)
or fertilizer-derived N (right), during the 56-d incubation. Data points represent the mean of observed values (n=4) and bars represent the
standard error of the mean but may be smaller than the symbols. Refer to section 2.3.1 and 2.3.2 for calculation of net mineralization.
151
APPENDIX C STATISTICAL TABLES FOR DATA IN CHAPTER 3
Table C.1 SAS Proc Mixed analysis of variance probability values of 0-30 cm SMN 2009 and 2010
after early and late broccoli harvest systems in autumn, discussed in section 3.4.2.
Fixed Effect
Pr > F
Treatment
0.0014
System
<.0001
Year
0.0009
System x Treatment
0.1034
Year x Treatment
0.8742
Year x System
<.0001
Year x System x Treatment
0.6567
Table C.2 SAS Proc Mixed analysis of variance probability values of 0-30 cm SMN 2009 after early
broccoli harvest in autumn, shown in Figure 3.1.
Fixed Effect
Pr > F
Treatment
0.0015
Sample Day
<.0001
Treatment x Sample Day
0.5139
152
Table C.3 SAS Proc Mixed analysis of variance probability values of 0-30 cm SMN 2010 after early
broccoli harvest in autumn, shown in Figure 3.1.
Fixed Effect
Pr > F
Treatment
0.0023
Sample Day
0.0006
Treatment x Sample Day
0.4768
Table C.4 SAS Proc Mixed analysis of variance probability values 0-30 cm SMN 2009 after late
broccoli harvest in autumn, shown in Figure 3.1.
Fixed Effect
Pr > F
Treatment
0.0423
Sample Day
n/a
Treatment x Sample Day
n/a
n/a
not available because only one sample day was collected
Table C.5 SAS Proc Mixed analysis of variance probability values 0-30 cm SMN 2010 after late
broccoli harvest in autumn, shown in Figure 3.1.
Fixed Effect
Pr > F
Treatment
0.0005
Sample Day
0.0660
Treatment x Sample Day
0.0630
153
Table C.6 SAS Proc Mixed analysis of variance probability values of SMN at spring wheat planting
2010 and 2011 after early and late broccoli harvest systems in 2009 and 2010, shown in Figure 3.1.
Fixed Effect
Pr > F
Treatment
0.0065
System
0.0834
Year
0.114
System x Treatment
0.8991
Year x Treatment
0.0049
Year x System
0.5833
Year x System x Treatment
0.0983
Treatment Effect
Pr > F
2010 early-harvest system
0.0067
2011 early-harvest system
0.3469
2010 late-harvest system
0.0075
2011 late-harvest system
0.0357
154
Table C.7 SAS Proc Mixed analysis of variance probability values of plant available N (0-90 cm
SMN, straw N, and grain N) at spring wheat harvest in 2010 and 2011 after early and late broccoli
harvest systems in 2009 and 2010, shown in Figure 3.1 and Figure 3.2.
Fixed Effect
Pr > F
Treatment
<.0001
System
0.7762
Year
<.0001
System x Treatment
0.6384
Year x Treatment
0.0031
Year x System
0.0014
Year x System x Treatment
0.0327
Treatment Effect
Pr > F
2010 early-harvest system
0.0101
2011 early-harvest system
0.0007
2010 late-harvest system
0.0065
2011 late-harvest system
0.6530
155
Table C.8 SAS Proc Mixed analysis of variance probability values of grain yield at spring wheat
harvest in 2010 and 2011 after early and late broccoli harvest systems in 2009 and 2010, shown in
Figure 3.3.
Effect
Pr > F
Treatment
<.0001
System
0.9973
Year
<.0001
System x Treatment
0.0745
Year x Treatment
0.3103
Year x System
0.8348
Year x System x Treatment
0.8437
Treatment Effect
Pr > F
2010 early-harvest system
0.0332
2011 early-harvest system
0.0012
2010 late-harvest system
0.0917
2011 late-harvest system
0.0010
156
Table C.9 SAS Proc Mixed analysis of variance probability values of plant biomass (straw and
grain) at spring wheat harvest in 2010and 2011 after early and late broccoli harvest systems in 2009
and 2010, shown in Figure 3.3.
Effect
Pr > F
Treatment
<.0001
System
0.1821
Year
<.0001
System x Treatment
0.1186
Year x Treatment
0.0188
Year x System
0.1706
Year x System x Treatment
0.6358
Treatment Effect
Pr > F
2010 early-harvest system
0.0798
2011 early-harvest system
0.0065
2010 late-harvest system
0.0677
2011 late-harvest system
0.0005
157
APPENDIX D STATISTICAL TABLES FOR DATA IN CHAPTER 4
Table D.1 SAS Proc Mixed analysis of variance probability values of fertilizer-derived N in broccoli
plant (head, stem, leaf) and soil pools (0-30 and 30-60 cm total, mineral, organic) at early and late
broccoli harvest systems 2011, shown in Table 4.2.
Fixed Effects
Fertilizer-derived N
Fertilizer-derived N
Total Stable Isotope N
(kg ha-1)
(% recovery)
(kg N ha-1)
Plant
System
0.3150
0.3147
0.1266
Organ
<0.0001
<0.0001
<0.0001
System x Organ
0.8615
0.8618
0.8929
System
0.0060
0.0060
0.0016
Pool
<0.0001
<0.0001
<0.0001
System x Pool
0.3607
0.3600
0.0063
Soil
158
Table D.2 SAS Proc Mixed analysis of variance probability values of 0-60 cm soil nitrate-N (NO3-N) and soil mineral N (SMN) after early and late broccoli harvest systems from 2011 to 2012
sampling, shown in Table 4.4.
Autumn after harvest
Spring wheat planting
Spring wheat harvest
Sept/Oct 2011
Apr 2012
July 2012
NO3--N
SMN
NO3--N
SMN
NO3--N
SMN
Treatment
0.0041
0.0046
0.4943
0.5318
0.0049
0.0070
System
0.0006
0.0008
0.0477
0.0564
0.2403
0.2132
System x Treatment
0.0571
0.0549
0.6532
0.6700
0.3522
0.3655
Sample Day
0.0954
0.0175
n/a
n/a
n/a
n/a
Sample Day x
0.5575
0.5721
n/a
n/a
n/a
n/a
0.1173
0.2906
n/a
n/a
n/a
n/a
0.8394
0.8525
n/a
n/a
n/a
n/a
Fixed Effects
Treatment
Sample Day x
System
Sample Day x
System x Treatment
n/a
not available because only one sample day was collected
159
Table D.3 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil Nderived from above-ground 15N enriched broccoli crop residue after early and late broccoli harvest
systems in autumn 2011, shown in Table 4.3.
Fixed Effects
-----------------------------------Crop Residue-Derived N (kg N ha-1)-----------------------------------
Sept/Oct 2011
Soil Total N
Soil Mineral N
Soil Microbial N
Soil Organic N
Treatment
0.2295
0.0001
n/a
0.5641
System
0.7789
0.1755
System x Treatment
0.6800
0.9189
Depth
0.0003
<0.0001
System x Depth
0.2837
0.1186
Treatment x Depth
0.6546
0.0020
System x Treatment x Depth
0.8933
0.4035
0-60 cm
0-30 cm
30-60 cm
0-30 cm
0-60 cm
Treatment
0.2885
0.0043
0.0641
0.0206
0.6501
System
0.7927
0.1548
0.6342
0.2329
0.9973
System x Treatment
0.7083
0.6347
0.2440
0.0734
0.7059
Treatment
0.3504
0.3162
n/a
0.7137
System
0.4089
0.05004
System x Treatment
0.4152
0.0042
Depth
0.0005
0.0003
System x Depth
0.0615
0.6970
Treatment x Depth
0.5723
0.8829
System x Treatment x Depth
0.7037
0.8128
0-60 cm
0-60 cm
0-30 cm
0-60 cm
Treatment
0.3617
0.4617
0.4196
0.3341
System
0.3960
0.6043
0.2037
0.4555
System x Treatment
0.4235
0.0465
0.1892
0.2892
n/a
n/a
n/a
n/a
n/a
n/a
0.7111
0.2303
0.0176
0.5157
0.5522
0.8957
Nov 2011
n/a
n/a
n/a
n/a
n/a
n/a
n/a
0.2068
0.8171
0.0002
0.0326
0.7259
0.5742
not available because microbial sampling only occurred in 0-30 cm depth Sept/Oct and Nov 2011.
160
Table D.4 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil N
recovered from above-ground 15N enriched broccoli crop residue after early and late broccoli
harvest systems in autumn 2011, shown in Figure 4.3.
Fixed Effects
--------------------------------Crop Residue-Derived N (% recovery)--------------------------------
Sept/Oct 2011
Soil Total N
Soil Mineral N
Soil Microbial N
Soil Organic N
Treatment
0.1917
<0.0001
n/a
0.3563
System
0.2434
0.0355
n/a
0.5118
System x Treatment
0.4643
0.4238
n/a
0.1241
Depth
<0.0001
<0.0001
n/a
0.0096
0.0736
0.0048
n/a
0.5561
Treatment x Depth
0.6719
<0.0001
n/a
0.6379
System x Treatment x Depth
0.6266
0.0778
n/a
0.9962
0-60 cm
0-30 cm
30-60 cm
0-30 cm
0-60 cm
Treatment
0.1940
0.0004
0.0687
0.0207
0.6214
System
0.2126
0.0306
0.3319
0.3025
0.4445
System x Treatment
0.4510
0.1547
0.2023
0.0880
0.5258
Treatment
0.2731
0.2233
n/a
0.6564
System
0.9103
0.5135
n/a
0.1380
System x Treatment
0.3647
0.0035
n/a
0.6564
Depth
0.0009
0.0011
n/a
<0.0001
System x Depth
0.1926
0.9038
n/a
0.0215
Treatment x Depth
0.6023
0.8865
n/a
0.7124
System x Treatment x Depth
0.8146
0.7344
n/a
0.4411
0-60 cm
0-60 cm
0-30 cm
0-60 cm
Treatment
0.2895
0.3884
0.5077
0.2451
System
0.9078
0.6054
0.2626
0.9789
System x Treatment
0.3762
0.0482
0.1512
0.2278
Nov 2011
n/a
not available because microbial sampling only occurred in 0-30 cm depth Sept/Oct and Nov 2011.
161
Table D.5 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil total N
(14N and 15N) in autumn 2011, after above-ground 15N enriched broccoli crop residue was
incorporated at early and late broccoli harvest systems, shown in Figure 4.3.
Fixed Effects
-------------------------------------Total Stable Isotope N (kg N ha-1)------------------------------------Soil Total N
Soil Mineral N
Soil Microbial N
Soil Organic N
0.3248
0.0070
n/a
0.6094
System
0.0078
0.1093
n/a
0.8116
System x Treatment
0.1016
0.0659
n/a
0.9632
Depth
<0.0001
<0.0001
n/a
0.0008
0.2507
0.0742
n/a
0.2826
Treatment x Depth
0.7409
0.0033
n/a
0.8503
System x Treatment x Depth
0.9825
0.1933
n/a
0.9964
0-60 cm
0-30 cm
30-60 cm
0-30 cm
0-60 cm
Treatment
0.4529
0.0188
0.4257
0.0175
0.4713
System
0.0139
0.0963
0.4186
0.0719
0.0137
System x Treatment
0.2204
0.1513
0.1732
0.0172
0.2278
Sept/Oct 2011
Treatment
Nov 2011
Treatment
0.5893
0.4188
n/a
0.0503
System
0.0921
0.6666
n/a
0.3651
System x Treatment
0.3131
0.1584
n/a
0.6607
Depth
<0.0001
0.0002
n/a
<0.0001
System x Depth
0.0356
0.8886
n/a
0.6741
Treatment x Depth
0.9285
0.6722
n/a
0.8778
System x Treatment x Depth
0.8596
0.7712
n/a
0.7842
0-60 cm
0-60 cm
0-30 cm
0-60 cm
Treatment
0.4641
0.5434
0.5347
0.4714
System
0.0292
0.7210
0.7955
0.0301
System x Treatment
0.1903
0.2956
0.4153
0.1884
n/a
not available because microbial sampling only occurred in 0-30 cm depth Sept/Oct and Nov 2011.
162
Table D.6 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil Nderived from 15N enriched broccoli crop residue after early and late broccoli harvest systems at
spring wheat planting (Apr) and harvest (Jul) in 2012, shown in Table 4.5.
Fixed Effects
---------------------------------Crop Residue-Derived N (kg N ha-1)--------------------------------
APRIL 2012
Soil Total N
Soil Mineral N
Soil Organic N
Treatment
0.2214
0.3366
0.1751
System
0.0162
0.0601
0.0246
System x Treatment
0.8995
0.7269
0.9195
Depth
0.0261
0.4206
0.0310
System x Depth
0.4712
0.1547
0.6046
Treatment x Depth
0.6187
0.8497
0.6495
System x Treatment x Depth
0.3759
0.8344
0.3720
0-60 cm
0-60 cm
0-60 cm
Treatment
0.3146
0.4140
0.2727
System
0.0250
0.0601
0.0386
System x Treatment
0.9375
0.7632
0.9591
Treatment
0.0847
0.0090
0.0876
System
0.5872
0.9466
0.5849
System x Treatment
0.7462
0.9084
0.7451
Depth
0.0014
<0.0001
0.0015
System x Depth
0.2262
0.7447
0.2209
Treatment x Depth
0.0498
0.0841
0.0500
System x Treatment x Depth
0.5657
0.7089
0.5651
JULY 2012
0-30 cm
30-60 cm
0-60 cm
0-30 cm
30-60 cm
Treatment
0.0673
0.7598
0.0435
0.0680
0.7490
System
0.3751
0.5131
0.9466
0.3697
0.5095
System x Treatment
0.8890
0.3341
0.9223
0.8891
0.3370
163
Table D.7 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil N
recovered from 15N enriched broccoli crop residue after early and late broccoli harvest systems at
spring wheat planting (Apr) and harvest (July) in 2012, shown in Table 4.5.
Fixed Effects
--------------------------------Crop Residue-Derived N (% recovery)--------------------------------
APRIL 2012
Soil Total N
Soil Mineral N
Soil Organic N
Treatment
0.1764
0.3397
0.1409
System
0.0305
0.1292
0.0423
System x Treatment
0.9665
0.8639
0.9683
Depth
0.0243
0.4096
0.0281
System x Depth
0.6103
0.1441
0.7468
Treatment x Depth
0.5592
0.8027
0.5888
System x Treatment x Depth
0.4128
0.7139
0.3977
0-60 cm
0-60 cm
0-60 cm
Treatment
0.2640
0.3150
0.2330
System
0.0444
0.1292
0.0627
System x Treatment
0.9891
0.8524
0.9995
Treatment
0.0784
0.0202
0.0805
System
0.2631
0.5059
0.2639
System x Treatment
0.5819
0.7995
0.5801
Depth
0.0017
<0.0001
0.0018
System x Depth
0.1388
0.7529
0.1361
Treatment x Depth
0.0530
0.1274
0.0530
System x Treatment x Depth
0.8233
0.8469
0.8232
0-60 cm
0-60 cm
0-60 cm
Treatment
0.1140
0.0857
0.1709
System
0.1792
0.5059
0.2813
System x Treatment
0.5827
0.8422
0.6528
JULY 2012
164
Table D.8 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil total N
(14N and 15N) at spring wheat planting (Apr) and harvest (July) 2012, after above-ground 15N
enriched broccoli crop residue was incorporated at early and late broccoli harvest systems, shown
in Table 4.5.
Fixed Effects
--------------------------------Total Stable Isotope N (kg N ha-1)--------------------------------
APRIL 2012
Soil Total N
Soil Mineral N
Soil Organic N
Treatment
0.8530
0.2760
0.8616
System
0.0651
0.2072
0.0668
System x Treatment
0.1636
0.9494
<0.0001
Depth
<0.0001
0.2869
0.1647
System x Depth
0.3252
0.0318
0.3365
Treatment x Depth
0.7373
0.6965
0.7409
System x Treatment x Depth
0.7813
0.7084
0.7797
0-60 cm
0-60 cm
0-60 cm
Treatment
0.4651
0.3154
0.4736
System
0.1302
0.2174
0.1335
System x Treatment
0.4427
0.9517
0.4457
Treatment
0.5191
0.2104
0.5282
System
0.0371
0.8523
0.0374
System x Treatment
0.5635
0.0833
0.5760
Depth
<0.0001
0.0010
<0.0001
System x Depth
0.2740
0.7724
0.2769
Treatment x Depth
0.9576
0.6000
0.9615
System x Treatment x Depth
0.1856
0.4852
0.1892
0-60 cm
0-60 cm
0-60 cm
Treatment
0.3502
0.2465
0.3601
System
0.0371
0.8540
0.0374
System x Treatment
0.3996
0.1199
0.4141
JULY 2012
165
Table D.9 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat yield or
biomass after above-ground 15N enriched broccoli crop residue was incorporated at early and late
broccoli harvest systems, shown in Table 4.6.
---------------Yield or Biomass (Mg ha-1)---------------
Fixed Effects
Grain
Straw
Total Plant
Treatment
0.3585
0.3397
0.6568
System
0.5321
0.2490
0.2878
System x Treatment
0.9545
0.8594
0.9081
Table D.10 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat N-derived
from broccoli crop residue after above-ground 15N enriched broccoli crop residue was incorporated
at early and late broccoli harvest systems, shown in Table 4.6.
Fixed Effects
--------------Crop Residue-Derived N (kg N ha-1)--------------Grain
Straw
Total Plant
Treatment
0.7702
0.3329
0.8901
System
0.0930
0.7297
0.2736
System x Treatment
0.9339
0.9090
0.9234
166
Table D.11 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat N
recovered from broccoli crop residue after above-ground 15N enriched broccoli crop residue was
incorporated at early and late broccoli harvest systems, shown in Table 4.6.
Fixed Effects
--------------Crop Residue-Derived N (% recovery)-------------Grain
Straw
Total Plant
Treatment
0.7780
0.2126
0.8031
System
0.5596
0.3305
0.9138
System x Treatment
0.7913
0.5425
0.6759
Table D.12 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat total N
(14N and 15N) after above-ground 15N enriched broccoli crop residue was incorporated at early and
late broccoli harvest systems, shown in Table 4.6.
Fixed Effects
---------------Total Stable Isotope N (kg N ha-1)-------------Grain
Straw
Total Plant
Treatment
0.4321
0.2037
0.9734
System
0.3489
0.0422
0.1099
System x Treatment
0.9065
0.5154
0.8472
167
Table D.13 SAS Proc Mixed analysis of variance probability values of spring wheat harvest
parameters in 2012, following early and late broccoli systems in the non-tracer trial, shown in Table
4.7.
------------------------------Harvest Parameters-----------------------------Grain N
Straw N
Total Plant Grain
Straw
Total Plant
(kg ha-1)
(kg ha-1)
N (kg ha-1)
Yield
Biomass
Biomass
(Mg ha-1)
(Mg ha-1)
(Mg ha-1)
Fixed Effects
Treatment
0.0799
0.0665
0.0179
0.0145
0.0573
0.0001
System
0.8967
0.4033
0.5787
0.6869
0.2105
0.4315
System x
0.9208
0.0447
0.2272
0.5520
0.0759
0.0034
Treatment
168
Table D.14 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil Nderived from 15N enriched residual fertilizer or broccoli roots after early and late broccoli harvest
systems in autumn (Sept to Nov 2011) , spring wheat planting (Apr 2011) and harvest (July 2012),
shown in Table 4.8.
Fixed Effects
-----------------------Residual Fertilizer or Root-Derived N (kg ha-1)----------------------
Sept/Oct 2011
Soil Total N
Soil Mineral N
Soil Organic N
Treatment
0.0887
0.7140
0.0847
System
0.1236
0.0712
0.3143
System x Treatment
0.6323
0.2875
0.8115
Depth
<0.0001
0.0011
<0.0001
System x Depth
0.0525
0.0232
0.1605
Treatment x Depth
0.0816
0.6267
0.1069
System x Treatment x Depth
0.2152
0.2262
0.5830
0-60 cm
0-60 cm
0-60 cm
Treatment
0.4073
0.6975
0.4506
System
0.1094
0.0712
0.2657
System x Treatment
0.5945
0.2751
0.9998
Treatment
0.3903
0.7901
0.6979
System
0.4415
0.5438
0.2031
System x Treatment
0.6773
0.0294
0.0844
Depth
<0.0001
0.0083
0.0002
System x Depth
0.4581
0.8665
0.6461
Treatment x Depth
0.6511
0.5040
0.9257
System x Treatment x Depth
0.9269
0.0681
0.2979
0-60 cm
0-60 cm
0-60 cm
Treatment
0.3953
0.6227
0.5781
System
0.4709
0.8028
0.3882
System x Treatment
0.7019
0.0948
0.3056
Nov 2011
169
April 2012
Treatment
0.8715
0.7616
0.8479
System
0.7350
0.8801
0.7336
System x Treatment
0.5118
0.0056
0.3704
Depth
<0.0001
0.4808
<0.0001
System x Depth
0.5005
0.8897
0.5088
Treatment x Depth
0.3475
0.1919
0.2973
System x Treatment x Depth
0.6918
0.2241
0.6157
0-60 cm
0-60 cm
0-60 cm
Treatment
0.9817
0.7453
0.5147
System
0.6733
0.8677
0.7151
System x Treatment
0.4998
0.0133
0.8582
Treatment
0.9014
0.5299
0.9055
System
0.1054
0.1715
0.1056
System x Treatment
0.8780
0.8893
0.8787
Depth
0.0001
<0.0001
0.0001
System x Depth
0.1156
0.5578
0.1142
Treatment x Depth
0.5516
0.7403
0.5453
System x Treatment x Depth
0.9916
0.4261
0.9998
0-60 cm
0-60 cm
0-60 cm
Treatment
0.8985
0.6109
0.1409
System
0.1268
0.2412
0.0194
System x Treatment
0.9055
0.9098
0.4203
July 2012
170
Table D.15 SAS Proc Mixed analysis of variance probability values of 0-30 and 30-60 cm soil total
N (14N and 15N) after early and late broccoli systems with 15N enriched residual fertilizer or broccoli
roots in autumn (Sept to Nov 2011) , spring wheat planting (Apr 2011) and harvest (July 2012),
shown in Table 4.8.
Fixed Effects
------------------------------------Total Stable Isotope N (kg N ha-1)------------------------------------
Soil Total N
Soil Mineral N
Soil Organic N
Treatment
0.7904
0.0248
0.8062
System
0.0055
0.0535
0.0059
System x Treatment
0.6696
0.7691
0.6415
Depth
<0.0001
<0.0001
<0.0001
System x Depth
0.7916
0.0737
0.7575
Treatment x Depth
0.3718
0.1589
0.3940
System x Treatment x Depth
0.4234
0.3622
0.4126
0-60 cm
0-60 cm
0-60 cm
Treatment
0.8269
0.0394
0.3642
System
0.0114
0.0535
0.0106
System x Treatment
0.7219
0.7604
0.5135
Treatment
0.8522
0.8699
0.8769
System
0.1631
0.7949
0.1760
System x Treatment
0.4552
0.0460
0.4509
Depth
0.0002
0.0047
0.0003
System x Depth
0.0699
0.8626
0.0713
Treatment x Depth
0.7682
0.5008
0.7965
System x Treatment x Depth
0.1385
0.0676
0.1366
0-60 cm
0-60 cm
0-60 cm
Treatment
0.7863
0.9012
0.0138
System
0.1769
0.7949
0.2751
Sept/Oct 2011
Nov 2011
171
System x Treatment
0.6850
0.1457
0.7434
Treatment
0.4184
0.9872
0.4185
System
0.0091
0.0325
0.0093
System x Treatment
0.5253
0.1696
0.5172
Depth
<0.0001
0.9346
<0.0001
System x Depth
0.9658
0.5958
0.9688
Treatment x Depth
0.8028
0.3367
0.7971
System x Treatment x Depth
0.7248
0.8226
0.7265
0-60 cm
0-60 cm
0-60 cm
Treatment
0.4600
0.9879
0.6280
System
0.0110
0.0352
0.0008
System x Treatment
0.5675
0.2110
0.8958
Treatment
0.1737
0.9618
0.1726
System
0.1409
0.0079
0.1353
System x Treatment
0.1834
0.8470
0.1819
Depth
<0.0001
0.1246
<0.0001
System x Depth
0.5739
0.2743
0.5697
Treatment x Depth
0.8941
0.2153
0.8896
System x Treatment x Depth
0.6456
0.9052
0.6450
0-60 cm
0-60 cm
0-60 cm
Treatment
0.1913
0.9681
0.8794
System
0.1346
0.0079
0.0809
System x Treatment
0.2003
0.8720
0.5609
April 2012
July 2012
172
Table D.16 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat yield or
biomass following the early and late broccoli systems with 15N enriched residual fertilizer or
broccoli root-derived N, shown in Table 4.9.
Fixed Effect
Grain Yield
Straw Biomass
Total Plant Biomass
(Mg ha-1)
(Mg ha-1)
(Mg ha-1)
Treatment
0.5752
0.8942
0.9648
System
0.1137
0.0501
0.0456
System x Treatment
0.7117
0.5851
0.5842
Table D.17 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat N derived
from 15N enriched residual fertilizer or broccoli root following the early and late broccoli systems,
shown in Table 4.9.
------------Residual Fertilizer or Root-Derived N (kg ha-1)---------Fixed Effect
Grain N
Treatment
0.5901
0.9883
0.7493
System
0.0464
0.0606
0.0347
System x Treatment
0.5948
0.6091
0.5994
Straw N
173
Total Plant N
Table D.18 SAS Proc Mixed analysis of variance probability values of 2012 spring wheat total N
(14N and 15N) after early and late broccoli systems with 15N enriched residual fertilizer or broccoli
roots, shown in Table 4.9.
---------------------Total Stable Isotope N (kg ha-1) --------------------Fixed Effect
Grain N
Treatment
0.7493
0.5793
0.9062
System
0.0583
0.0631
0.0446
System x Treatment
0.7474
0.1668
0.3432
Straw N
174
Total Plant N